Wake-and-go mechanism with system address bus transaction master

ABSTRACT

A wake-and-go mechanism is provided for a data processing system. The wake-and-go mechanism is configured to issue a look-ahead load command on a system bus to read a data value from a target address and perform a comparison operation to determine whether the data value at the target address indicates that an event for which a thread is waiting has occurred. In response to the comparison resulting in a determination that the event has not occurred, the wake-and-go engine populates the wake-and-go storage array with the target address and snoops the target address on the system bus.

This invention was made with United States Government support underAgreement No. HR0011-07-9-0002 awarded by DARPA. The Government hascertain rights in the invention.

BACKGROUND

1. Technical Field

The present application relates generally to an improved data processingsystem and method. More specifically, the present application isdirected to a mechanism to wake a sleeping thread based on anasynchronous event.

2. Description of Related Art

Multithreading is multitasking within a single program. Multithreadingallows multiple streams of execution to take place concurrently withinthe same program, each stream processing a different transaction ormessage. In order for a multithreaded program to achieve trueperformance gains, it must be run in a multitasking or multiprocessingenvironment, which allows multiple operations to take place.

Certain types of applications lend themselves to multithreading. Forexample, in an order processing system, each order can be enteredindependently of the other orders. In an image editing program, acalculation-intensive filter can be performed on one image, while theuser works on another. Multithreading is also used to createsynchronized audio and video applications.

In addition, a symmetric multiprocessing (SMP) operating system usesmultithreading to allow multiple CPUs to be controlled at the same time.An SMP computing system is a multiprocessing architecture in whichmultiple central processing units (CPUs) share the same memory. SMPspeeds up whatever processes can be overlapped. For example, in adesktop computer, SMP may speed up the running of multiple applicationssimultaneously. If an application is multithreaded, which allows forconcurrent operations within the application itself, then SMP mayimprove the performance of that single application.

If a process, or thread, is waiting for an event, then the process goesto sleep. A process is said to be “sleeping,” if the process is in aninactive state. The thread remains in memory, but is not queued forprocessing until an event occurs. Typically, this event is detected whenthere is a change to a value at a particular address or when there is aninterrupt.

As an example of the latter, a processor may be executing a firstthread, which goes to sleep. The processor may then begin executing asecond thread. When an interrupt occurs, indicating that an event forwhich the first thread was waiting, the processor may then stop runningthe second thread and “wake” the first thread. However, in order toreceive the interrupt, the processor must perform interrupt eventhandling, which is highly software intensive. An interrupt handler hasmultiple levels, typically including a first level interrupt handler(FLIH) and a second level interrupt handler (SLIH); therefore, interrupthandling may be time-consuming.

In the former case, the processor may simply allow the first thread toperiodically poll a memory location to determine whether a particularevent occurs. The first thread performs a get instruction and a compareinstruction (GET&CMP) to determine whether a value at a given address ischanged to an expected value. When one considers that a computing systemmay be running thousands of threads, many of which are waiting for anevent at any given time, there are many wasted processor cycles spentpolling memory locations when an expected event has not occurred.

SUMMARY

In one illustrative embodiment, a method, in a data processing system,performs a look-ahead operation in a wake-and-go mechanism. The methodcomprises issuing a look-ahead load command on a system bus to read adata value from a target address. The method comprises performing acomparison operation to determine whether the data value at the targetaddress indicates that an event for which a thread is waiting hasoccurred. In response to the comparison resulting in a determinationthat the event has not occurred, the method populates a wake-and-gostorage array with the target address. The method further comprisessnooping the target address on the system bus.

In another illustrative embodiment, a data processing system comprises awake-and-go mechanism and a wake-and-go storage array. The wake-and-gomechanism is configured to issue a look-ahead load command on a systembus to read a data value from a target address. The wake-and-gomechanism is configured to perform a comparison operation to determinewhether the data value at the target address indicates that an event forwhich a thread is waiting has occurred. In response to the comparisonresulting in a determination that the event has not occurred, thewake-and-go mechanism is configured to populate the wake-and-go storagearray with the target address. The wake-and-go mechanism is furtherconfigured to snoop the target address on the system bus.

In another illustrative embodiment, a computer program product comprisesa computer useable medium having a computer readable program. Thecomputer readable program, when executed on a computing device, causesthe computing device to issue a look-ahead load command on a system busto read a data value from a target address. The computer readableprogram causes the computing device to perform a comparison operation todetermine whether the data value at the target address indicates that anevent for which a thread is waiting has occurred. In response to thecomparison resulting in a determination that the event has not occurred,the computer readable program causes the computing device to populate awake-and-go storage array with the target address. The computer readableprogram further causes the computing device to snoop the target addresson the system bus.

These and other features and advantages of the present invention will bedescribed in, or will become apparent to those of ordinary skill in theart in view of, the following detailed description of the exemplaryembodiments of the present invention.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention, as well as a preferred mode of use and further objectivesand advantages thereof, will best be understood by reference to thefollowing detailed description of illustrative embodiments when read inconjunction with the accompanying drawings, wherein:

FIG. 1 is a block diagram of an exemplary data processing system inwhich aspects of the illustrative embodiments may be implemented;

FIG. 2 is a block diagram of a wake-and-go mechanism in a dataprocessing system in accordance with an illustrative embodiment;

FIG. 3 is a block diagram of a wake-and-go mechanism with a hardwareprivate array in accordance with an illustrative embodiment;

FIGS. 4A and 4B are block diagrams illustrating operation of awake-and-go mechanism with specialized processor instructions inaccordance with an illustrative embodiment;

FIGS. 5A and 5B are block diagrams illustrating operation of awake-and-go mechanism with a specialized operating system call inaccordance with an illustrative embodiment;

FIG. 6 is a block diagram illustrating operation of a wake-and-gomechanism with a background sleeper thread in accordance with anillustrative embodiment;

FIGS. 7A and 7B are flowcharts illustrating operation of a wake-and-gomechanism in accordance with the illustrative embodiments;

FIGS. 8A and 8B are flowcharts illustrating operation of a wake-and-gomechanism with prioritization of threads in accordance with theillustrative embodiments;

FIGS. 9A and 9B are flowcharts illustrating operation of a wake-and-gomechanism with dynamic allocation in a hardware private array inaccordance with the illustrative embodiments;

FIG. 10 is a block diagram of a hardware wake-and-go mechanism in a dataprocessing system in accordance with an illustrative embodiment;

FIGS. 11A and 11B illustrate a series of instructions that are aprogramming idiom for wake-and-go in accordance with an illustrativeembodiment;

FIGS. 12A and 12B are block diagrams illustrating operation of ahardware wake-and-go mechanism in accordance with an illustrativeembodiment;

FIGS. 13A and 13B are flowcharts illustrating operation of a hardwarewake-and-go mechanism in accordance with the illustrative embodiments;

FIGS. 14A and 14B are block diagrams illustrating operation of awake-and-go engine with look-ahead in accordance with an illustrativeembodiment;

FIG. 15 is a flowchart illustrating a look-ahead polling operation of awake-and-go look-ahead engine in accordance with an illustrativeembodiment;

FIG. 16 is a block diagram illustrating operation of a wake-and-gomechanism with speculative execution in accordance with an illustrativeembodiment;

FIG. 17 is a flowchart illustrating operation of a look-aheadwake-and-go mechanism with speculative execution in accordance with anillustrative embodiment;

FIGS. 18A and 18B are flowcharts illustrating operation of a wake-and-gomechanism with speculative execution during execution of a thread inaccordance with an illustrative embodiment;

FIG. 19 is a block diagram illustrating data monitoring in a multipleprocessor system in accordance with an illustrative embodiment;

FIG. 20 is a block diagram illustrating operation of a wake-and-gomechanism in accordance with an illustrative embodiment;

FIGS. 21A and 21B are block diagrams illustrating parallel lock spinningusing a wake-and-go mechanism in accordance with an illustrativeembodiment;

FIGS. 22A and 22B are flowcharts illustrating parallel lock spinningusing a wake-and-go mechanism in accordance with the illustrativeembodiments;

FIG. 23 is a block diagram illustrating a wake-and-go engine with acentral repository wake-and-go array in a multiple processor system inaccordance with an illustrative embodiment;

FIG. 24 illustrates a central repository wake-and-go-array in accordancewith an illustrative embodiment;

FIG. 25 is a block diagram illustrating a programming idiom acceleratorin accordance with an illustrative embodiment;

FIG. 26 is a series of instructions that are a programming idiom withprogramming language exposure in accordance with an illustrativeembodiment;

FIG. 27 is a block diagram illustrating a compiler that exposesprogramming idioms in accordance with an illustrative embodiment;

FIG. 28 is a flowchart illustrating operation of a compiler exposingprogramming idioms in accordance with an illustrative embodiment;

FIG. 29 is a flowchart illustrating operation of a wake-and-go engineperforming look-ahead polling with system bus response in accordancewith an illustrative embodiment;

FIG. 30 is a flowchart illustrating operation of a wake-and-go engineperforming a snoop operation without data exclusivity in accordance withan illustrative embodiment; and

FIG. 31 is a flowchart illustrating operation of snoop response logic onthe system bus in accordance with an illustrative embodiment.

DETAILED DESCRIPTION OF THE ILLUSTRATIVE EMBODIMENTS

With reference now to the figures and in particular with reference toFIG. 1, an exemplary diagram of data processing environments is providedin which illustrative embodiments of the present invention may beimplemented. It should be appreciated that FIG. 1 is only exemplary andis not intended to assert or imply any limitation with regard to theenvironments in which aspects or embodiments of the present inventionmay be implemented. Many modifications to the depicted environments maybe made without departing from the spirit and scope of the presentinvention.

FIG. 1 is a block diagram of an exemplary data processing system inwhich aspects of the illustrative embodiments may be implemented. Asshown, data processing system 100 includes processor cards 111 a-111 n.Each of processor cards 111 a-111 n includes a processor and a cachememory. For example, processor card 111 a contains processor 112 a andcache memory 113 a, and processor card 111 n contains processor 112 nand cache memory 113 n.

Processor cards 111 a-111 n connect to symmetric multiprocessing (SMP)bus 115. SMP bus 115 supports a system planar 120 that containsprocessor cards 111 a-111 n and memory cards 123. The system planar alsocontains data switch 121 and memory controller/cache 122. Memorycontroller/cache 122 supports memory cards 123 that includes localmemory 116 having multiple dual in-line memory modules (DIMMs).

Data switch 121 connects to bus bridge 117 and bus bridge 118 locatedwithin a native I/O (NIO) planar 124. As shown, bus bridge 118 connectsto peripheral components interconnect (PCI) bridges 125 and 126 viasystem bus 119. PCI bridge 125 connects to a variety of I/O devices viaPCI bus 128. As shown, hard disk 136 may be connected to PCI bus 128 viasmall computer system interface (SCSI) host adapter 130. A graphicsadapter 131 may be directly or indirectly connected to PCI bus 128. PCIbridge 126 provides connections for external data streams throughnetwork adapter 134 and adapter card slots 135 a-135 n via PCI bus 127.

An industry standard architecture (ISA) bus 129 connects to PCI bus 128via ISA bridge 132. ISA bridge 132 provides interconnection capabilitiesthrough NIO controller 133 having serial connections Serial 1 and Serial2. A floppy drive connection 137, keyboard connection 138, and mouseconnection 139 are provided by NIO controller 133 to allow dataprocessing system 100 to accept data input from a user via acorresponding input device. In addition, non-volatile RAM (NVRAM) 140provides a non-volatile memory for preserving certain types of data fromsystem disruptions or system failures, such as power supply problems. Asystem firmware 141 also connects to ISA bus 129 for implementing theinitial Basic Input/Output System (BIOS) functions. A service processor144 connects to ISA bus 129 to provide functionality for systemdiagnostics or system servicing.

The operating system (OS) resides on hard disk 136, which may alsoprovide storage for additional application software for execution bydata processing system. NVRAM 140 stores system variables and errorinformation for field replaceable unit (FRU) isolation. During systemstartup, the bootstrap program loads the operating system and initiatesexecution of the operating system. To load the operating system, thebootstrap program first locates an operating system kernel type fromhard disk 136, loads the OS into memory, and jumps to an initial addressprovided by the operating system kernel. Typically, the operating systemloads into random-access memory (RAM) within the data processing system.Once loaded and initialized, the operating system controls the executionof programs and may provide services such as resource allocation,scheduling, input/output control, and data management.

The present invention may be executed in a variety of data processingsystems utilizing a number of different hardware configurations andsoftware such as bootstrap programs and operating systems. The dataprocessing system 100 may be, for example, a stand-alone system or partof a network such as a local-area network (LAN) or a wide-area network(WAN).

FIG. 1 is an example of a symmetric multiprocessing (SMP) dataprocessing system in which processors communicate via a SMP bus 115.FIG. 1 is only exemplary and is not intended to assert or imply anylimitation with regard to the environments in which aspects orembodiments of the present invention may be implemented. The depictedenvironments may be implemented in other data processing environmentswithout departing from the spirit and scope of the present invention.

FIG. 2 is a block diagram of a wake-and-go mechanism in a dataprocessing system in accordance with an illustrative embodiment. Threads202, 204, 206 run on one or more processors (not shown). Threads 202,204, 206 make calls to operating system 210 and application programminginterface (API) 212 to communicate with each other, memory 232 via bus220, or other devices within the data processing system.

In accordance with the illustrative embodiment, a wake-and-go mechanismfor a microprocessor includes wake-and-go array 222 attached to the SMPfabric. The SMP fabric is a communication medium through whichprocessors communicate. The SMP fabric may comprise a single SMP bus ora system of busses, for example. In the depicted example, the SMP fabriccomprises bus 220. A thread, such as thread 202, for example, mayinclude instructions that indicate that the thread is waiting for anevent. The event may be an asynchronous event, which is an event thathappens independently in time with respect to execution of the thread inthe data processing system. For example, an asynchronous event may be atemperature value reaching a particular threshold, a stock price fallingbelow a given threshold, or the like. Alternatively, the event may berelated in some way to execution of the thread. For example, the eventmay be obtaining a lock for exclusive access to a database record or thelike.

Typically, the instructions may comprise a series of get-and-comparesequences; however, in accordance with the illustrative embodiment, theinstructions include instructions, calls to operating system 210 or API212, or calls to a background sleeper thread, such as thread 204, forexample, to update wake-and-go array 222. These instructions store atarget address in wake-and-go array 222, where the event the thread iswaiting for is associated with the target address. After updatingwake-and-go array 222 with the target address, thread 202 may go tosleep.

When thread 202 goes to sleep, operating system 210 or other software orhardware saves the state of thread 202 in thread state storage 234,which may be allocated from memory 232 or may be a hardware privatearray within the processor (not shown) or pervasive logic (not shown).When a thread is put to sleep, i.e., removed from the run queue of aprocessor, the operating system must store sufficient information on itsoperating state such that when the thread is again scheduled to run onthe processor, the thread can resume operation from an identicalposition. This state information is sometime referred to as the thread's“context.” The state information may include, for example, addressspace, stack space, virtual address space, program counter, instructionregister, program status word, and the like.

If a transaction appears on bus 220 that modifies a value at an addressin wake-and-go array 222, then operating system 210 may wake thread 202.Operating system 210 wakes thread 202 by recovering the state of thread202 from thread state storage 234. Thread 202 may then determine whetherthe transaction corresponds to the event for which the thread waswaiting by performing a get-and-compare operation, for instance. If thetransaction is the event for which the thread was waiting, then thread202 will perform work. However, if the transaction is not the event,then thread 202 will go back to sleep. Thus, thread 202 only performs aget-and-compare operation if there is a transaction that modifies thetarget address.

Alternatively, operating system 210 or a background sleeper thread, suchas thread 204, may determine whether the transaction is the event forwhich the thread was waiting. Before being put to sleep, thread 202 mayupdate a data structure in the operating system or background sleeperthread with a value for which it is waiting.

In one exemplary embodiment, wake-and-go array 222 may be a contentaddressable memory (CAM). A CAM is a special type of computer memoryoften used in very high speed searching applications. A CAM is alsoknown as associative memory, associative storage, or associative array,although the last term is more often used for a programming datastructure. Unlike a random access memory (RAM) in which the usersupplies a memory address and the RAM returns the data value stored atthat address, a CAM is designed such that the user supplies a data valueand the CAM searches its entire memory to see if that data value isstored within the CAM. If the data value is found, the CAM returns alist of one or more storage addresses where the data value was found. Insome architectures, a CAM may return the data value or other associatedpieces of data. Thus, a CAM may be considered the hardware embodiment ofwhat in software terms would be called an associative array.

Thus, in the exemplary embodiment, wake-and-go array 222 may comprise aCAM and associated logic that will be triggered if a transaction appearson bus 220 that modifies an address stored in the CAM. A transactionthat modifies a value at a target address may be referred to as a“kill”; thus, wake-and-go array 222 may be said to be “snooping kills.”In this exemplary embodiment, the data values stored in the CAM are thetarget addresses at which threads are waiting for something to bewritten. The address at which a data value, a given target address, isstored is referred to herein as the storage address. Each storageaddress may refer to a thread that is asleep and waiting for an event.Wake-and-go array 222 may store multiple instances of the same targetaddress, each instance being associated with a different thread waitingfor an event at that target address. Thus, when wake-and-go array 222snoops a kill at a given target address, wake-and-go array 222 mayreturn one or more storage addresses that are associated with one ormore sleeping threads.

In one exemplary embodiment, software may save the state of thread 202,for example. The state of a thread may be about 1000 bytes, for example.Thread 202 is then put to sleep. When wake-and-go array 222 snoops akill at a given target address, logic associated with wake-and-go array222 may generate an exception. The processor that was running thread 202sees the exception and performs a trap. A trap is a type of synchronousinterrupt typically caused by an exception condition, in this case akill at a target address in wake-and-go array 222. The trap may resultin a switch to kernel mode, wherein the operating system 210 performssome action before returning control to the originating process. In thiscase, the trap results in other software, such as operating system 210,for example, to reload thread 202 from thread state storage 234 and tocontinue processing of the active threads on the processor.

FIG. 3 is a block diagram of a wake-and-go mechanism with a hardwareprivate array in accordance with an illustrative embodiment. Threads302, 304, 306 run on processor 300. Threads 302, 304, 306 make calls tooperating system 310 and application programming interface (API) 312 tocommunicate with each other, memory 332 via bus 320, or other deviceswithin the data processing system. While the data processing system inFIG. 3 shows one processor, more processors may be present dependingupon the implementation where each processor has a separate wake-and-goarray or one wake-and-go array stores target addresses for threads formultiple processors.

In an illustrative embodiment, when a thread, such as thread 302, firststarts executing, a wake-and-go mechanism automatically allocates spacefor thread state in hardware private array 308 and space for a targetaddress and other information, if any, in wake-and-go array 322.Allocating space may comprise reserving an address range in a memory,such as a static random access memory, that is hidden in hardware, suchas processor 300, for example. Alternatively, if hardware private array308 comprises a reserved portion of system memory, such as memory 332,then the wake-and-go mechanism may request a sufficient portion ofmemory, such as 1000 bytes, for example, to store thread state for thatthread.

Thus hardware private array 308 may be a memory the size of whichmatches the size of thread state information for all running threads.When a thread ends execution and is no longer in the run queue ofprocessor 300, the wake-and-go mechanism de-allocates the space for thethread state information for that thread.

In accordance with the illustrative embodiment, a wake-and-go mechanismfor a microprocessor includes wake-and-go array 322 attached to the SMPfabric. The SMP fabric is a communication medium through whichprocessors communicate. The SMP fabric may comprise a single SMP bus ora system of busses, for example. In the depicted example, the SMP fabriccomprises bus 320. A thread, such as thread 302, for example, mayinclude instructions that indicate that the thread is waiting for anevent. The event may be an asynchronous event, which is an event thathappens independently in time with respect to execution of the thread inthe data processing system. For example, an asynchronous event may be atemperature value reaching a particular threshold, a stock price fallingbelow a given threshold, or the like. Alternatively, the event may berelated in some way to execution of the thread. For example, the eventmay be obtaining a lock for exclusive access to a database record or thelike.

Typically, the instructions may comprise a series of get-and-comparesequences; however, in accordance with the illustrative embodiment, theinstructions include instructions, calls to operating system 310 or API312, or calls to a background sleeper thread, such as thread 304, forexample, to update wake-and-go array 322. These instructions store atarget address in wake-and-go array 322, where the event the thread iswaiting for is associated with the target address. After updatingwake-and-go array 322 with the target address, thread 302 may go tosleep.

When thread 302 goes to sleep, operating system 310 or other software orhardware within processor 300 saves the state of thread 302 in hardwareprivate array 308 within processor 300. In an alternative embodiment,hardware private array may be embodied within pervasive logic associatedwith bus 320 or wake-and-go array 322. When a thread is put to sleep,i.e., removed from the run queue of processor 300, operating system 310must store sufficient information on its operating state such that whenthe thread is again scheduled to run on processor 300, the thread canresume operation from an identical position. This state information issometime referred to as the thread's “context.” The state informationmay include, for example, address space, stack space, virtual addressspace, program counter, instruction register, program status word, andthe like, which may comprise about 1000 bytes, for example.

If a transaction appears on bus 320 that modifies a value at an addressin wake-and-go array 322, then operating system 310 may wake thread 302.Operating system 310 wakes thread 302 by recovering the state of thread302 from hardware private array 308. Thread 302 may then determinewhether the transaction corresponds to the event for which the threadwas waiting by performing a get-and-compare operation, for instance. Ifthe transaction is the event for which the thread was waiting, thenthread 302 will perform work. However, if the transaction is not theevent, then thread 302 will go back to sleep. Thus, thread 302 onlyperforms a get-and-compare operation if there is a transaction thatmodifies the target address.

Hardware private array 308 is a thread state storage that is embeddedwithin processor 300 or within logic associated with bus 320 orwake-and-go array 322. Hardware private array 308 may be a memorystructure, such as a static random access memory (SRAM), which isdedicated to storing thread state for sleeping threads that have atarget address in wake-and-go array 322. In an alternative embodiment,hardware private array 308 may be a hidden area of memory 332. Hardwareprivate array 308 is private because it cannot be addressed by theoperating system or work threads.

Hardware private array 308 and/or wake-and-go array 322 may have alimited storage area. Therefore, each thread may have an associatedpriority. The wake-and-go mechanism described herein may store thepriority of sleeping threads with the thread state in hardware privatearray 308. Alternatively, the wake-and-go mechanism may store thepriority with the target address in wake-and-go array 322. When athread, such as thread 302, for example, goes to sleep, the wake-and-gomechanism may determine whether there is sufficient room to store thethread state of thread 302 in hardware private array 308. If there issufficient space, then the wake-and-go mechanism simply stores thethread state in hardware private array 308.

If there is insufficient space in hardware private array 308, then ifthe hardware private array is a portion of system memory 332, then thewake-and-go mechanism may ask for more of system memory 332 to beallocated to the hardware private array 308.

If there is insufficient space in hardware private array 308, then thewake-and-go mechanism may compare the priority of thread 302 to thepriorities of the threads already stored in hardware private array 308and wake-and-go array 322. If thread 302 has a lower priority than allof the threads already stored in hardware private array 208 andwake-and-go array 322, then thread 302 may default to a flee model, suchas polling or interrupt as in the prior art. If thread 302 has a higherpriority than at least one thread already stored in hardware privatearray 308 and wake-and-go array 322, then the wake-and-go mechanism may“punt” a lowest priority thread, meaning the thread is removed fromhardware private array 308 and wake-and-go array 322 and converted to aflee model.

In an alternative embodiment, priority may be determined by otherfactors. For example, priority may be time driven. That is, thewake-and-go mechanism may simply punt the stalest thread in hardwareprivate array 308 and wake-and-go array 322.

Alternatively, operating system 310 or a background sleeper thread, suchas thread 304, may determine whether the transaction is the event forwhich the thread was waiting. Before being put to sleep, thread 302 mayupdate a data structure in the operating system or background sleeperthread with a value for which it is waiting.

In one exemplary embodiment, wake-and-go array 322 may be a contentaddressable memory (CAM). A CAM is a special type of computer memoryoften used in very high speed searching applications. A CAM is alsoknown as associative memory, associative storage, or associative array,although the last term is more often used for a programming datastructure. Unlike a random access memory (RAM) in which the usersupplies a memory address and the RAM returns the data value stored atthat address, a CAM is designed such that the user supplies a data valueand the CAM searches its entire memory to see if that data value isstored within the CAM. If the data value is found, the CAM returns alist of one or more storage addresses where the data value was found. Insome architectures, a CAM may return the data value or other associatedpieces of data. Thus, a CAM may be considered the hardware embodiment ofwhat in software terms would be called an associative array.

Thus, in the exemplary embodiment, wake-and-go array 322 may comprise aCAM and associated logic that will be triggered if a transaction appearson bus 320 that modifies an address stored in the CAM. A transactionthat modifies a value at a target address may be referred to as a“kill”; thus, wake-and-go array 322 may be said to be “snooping kills.”In this exemplary embodiment, the data values stored in the CAM are thetarget addresses at which threads are waiting for something to bewritten. The address at which a data value, a given target address, isstored is referred to herein as the storage address. Each storageaddress may refer to a thread that is asleep and waiting for an event.Wake-and-go array 322 may store multiple instances of the same targetaddress, each instance being associated with a different thread waitingfor an event at that target address. Thus, when wake-and-go array 322snoops a kill at a given target address, wake-and-go array 322 mayreturn one or more storage addresses that are associated with one ormore sleeping threads.

FIGS. 4A and 4B are block diagrams illustrating operation of awake-and-go mechanism with specialized processor instructions inaccordance with an illustrative embodiment. With particular reference toFIG. 4A, thread 410 runs in a processor (not shown) and performs somework. Thread 410 executes a specialized processor instruction to updatewake-and-go array 422, storing a target address A₂ in array 422. Then,thread 410 goes to sleep with thread state being stored in thread statestorage 412.

When a transaction appears on SMP fabric 420 with an address thatmatches the target address A₂, array 422 returns the storage addressthat is associated with thread 410. The operating system (not shown) orsome other hardware or software then wakes thread 410 by retrieving thethread state information from thread state storage 412 and placing thethread in the run queue for the processor. Thread 410 may then perform acompare-and-branch operation to determine whether the value written tothe target address represents the event for which thread 410 is waiting.In the depicted example, the value written to the target address doesnot represent the event for which thread 410 is waiting; therefore,thread 410 goes back to sleep.

In one exemplary embodiment, software may save the state of thread 410,for example. Thread 410 is then put to sleep. When wake-and-go array 422snoops a kill at target address A₂, logic associated with wake-and-goarray 422 may generate an exception. The processor sees the exceptionand performs a trap, which results in a switch to kernel mode, whereinthe operating system may perform some action before returning control tothe originating process. In this case, the trap results in othersoftware to reload thread 410 from thread state storage 412 and tocontinue processing of the active threads on the processor.

In one exemplary embodiment, thread state storage 412 is a hardwareprivate array. Thread state storage 412 is a memory that is embeddedwithin the processor or within logic associated with bus 420 orwake-and-go array 422. Thread state storage 412 may comprise memorycells that are dedicated to storing thread state for sleeping threadsthat have a target address in wake-and-go array 422. In an alternativeembodiment, thread state storage 412 may be a hidden area of memory 332,for example. Thread state storage 412 may private in that it cannot beaddressed by the operating system or work threads.

Turning to FIG. 4B, thread 410 runs in a processor (not shown) andperforms some work. Thread 410 executes a specialized processorinstruction to update wake-and-go array 422, storing a target address A₂in array 422. Then, thread 410 goes to sleep with thread state beingstored in thread state storage 412.

When a transaction appears on SMP fabric 420 with an address thatmatches the target address A₂, array 422 returns the storage addressthat is associated with thread 410. The operating system (not shown) orsome other hardware or software then wakes thread 410 by retrieving thethread state information from thread state storage 412 and placing thethread in the run queue for the processor. Thread 410 may then perform acompare-and-branch operation to determine whether the value written tothe target address represents the event for which thread 410 is waiting.In the depicted example, the value written to the target address doesrepresent the event for which thread 410 is waiting; therefore, thread410 updates the array to remove the target address from array 422, andperforms more work.

FIGS. 5A and 5B are block diagrams illustrating operation of awake-and-go mechanism with a specialized operating system call inaccordance with an illustrative embodiment. With particular reference toFIG. 5A, thread 510 runs in a processor (not shown) and performs somework. Thread 510 makes a call to operating system 530 to updatewake-and-go array 522. The call to operating system 530 may be anoperating system call or a call to an application programming interface(not shown) provided by operating system 530. Operating system 530 thenstores a target address A₂ in array 522. Then, thread 510 goes to sleepwith thread state being stored in thread state storage 512.

When a transaction appears on SMP fabric 520 with an address thatmatches the target address A₂, array 522 returns the storage addressthat is associated with thread 510. Operating system 530 or some otherhardware or software then wakes thread 510 by retrieving the threadstate information from thread state storage 512 and placing the threadin the run queue for the processor. Thread 510 may then perform acompare-and-branch operation to determine whether the value written tothe target address represents the event for which thread 510 is waiting.In the depicted example, the value written to the target address doesnot represent the event for which thread 510 is waiting; therefore,thread 510 goes back to sleep.

In one exemplary embodiment, software may save the state of thread 510,for example. Thread 510 is then put to sleep. When wake-and-go array 522snoops a kill at target address A₂, logic associated with wake-and-goarray 522 may generate an exception. The processor sees the exceptionand performs a trap, which results in a switch to kernel mode, whereinoperating system 530 may perform some action before returning control tothe originating process. In this case, the trap results in the operatingsystem 530 to reload thread 510 from thread state storage 512 and tocontinue processing of the active threads on the processor.

In one exemplary embodiment, thread state storage 512 is a hardwareprivate array. Thread state storage 512 is a memory that is embeddedwithin the processor or within logic associated with bus 520 orwake-and-go array 522. Thread state storage 512 may comprise memorycells that are dedicated to storing thread state for sleeping threadsthat have a target address in wake-and-go array 522. In an alternativeembodiment, thread state storage 512 may be a hidden area of memory 332,for example. Thread state storage 512 may private in that it cannot beaddressed by the operating system or work threads.

Turning to FIG. 5B, thread 510 runs in a processor (not shown) andperforms some work. Thread 510 makes a call to operating system 530 toupdate wake-and-go array 522. The call to operating system 530 may be anoperating system call or a call to an application programming interface(not shown) provided by operating system 530. Operating system 530 thenstores a target address A₂ in array 522. Then, thread 510 goes to sleepwith thread state being stored in thread state storage 512.

When a transaction appears on SMP fabric 520 with an address thatmatches the target address A₂, array 522 returns the storage addressthat is associated with thread 510. Operating system 530 or some otherhardware or software then wakes thread 510 by retrieving the threadstate information from thread state storage 512 and placing the threadin the run queue for the processor. Thread 510 may then perform acompare-and-branch operation to determine whether the value written tothe target address represents the event for which thread 510 is waiting.In the depicted example, the value written to the target address doesrepresent the event for which thread 510 is waiting; therefore, thread510 updates the array to remove the target address from array 522, andperforms more work.

FIG. 6 is a block diagram illustrating operation of a wake-and-gomechanism with a background sleeper thread in accordance with anillustrative embodiment. Thread 610 runs in a processor (not shown) andperforms some work. Thread 610 makes a call to background sleeper thread640 to update wake-and-go array 622. The call to background sleeperthread 640 may be a remote procedure call, for example, or a call to anapplication programming interface (not shown) provided by backgroundsleeper thread 640. Background sleeper thread 640 then stores a targetaddress A₂ in array 622. Thread 610 may also store other information inassociation with background sleeper thread 640, such as a value forwhich thread 610 is waiting to be written to target address A₂. Then,thread 610 goes to sleep with thread state being stored in thread statestorage 612.

When a transaction appears on SMP fabric 620 with an address thatmatches the target address A₂, array 622 returns the storage addressthat is associated with thread 610. Operating system 630 or some otherhardware or software then wakes thread 610 by retrieving the threadstate information from thread state storage 612 and placing the threadin the run queue for the processor. Background sleeper thread 640 maythen perform a compare-and-branch operation to determine whether thevalue written to the target address represents the event for whichthread 610 is waiting. If the value written to the target address doesrepresent the event for which thread 610 is waiting, then backgroundsleeper thread 640 does nothing. However, if the value written to thetarget address does represent the event for which thread 610 is waiting,then background sleeper thread 640 wakes thread 640. Thereafter, thread610 updates the array 622 to remove the target address from array 622and performs more work.

In one exemplary embodiment, software may save the state of thread 610,for example. Thread 610 is then put to sleep. When wake-and-go array 622snoops a kill at target address A₂, logic associated with wake-and-goarray 622 may generate an exception. The processor sees the exceptionand performs a trap, which results in a switch to kernel mode, whereinthe operating system may perform some action before returning control tothe originating process. In this case, the trap results in othersoftware, such as background sleeper thread 640 to reload thread 610from thread state storage 612 and to continue processing of the activethreads on the processor.

In one exemplary embodiment, thread state storage 612 is a hardwareprivate array. Thread state storage 612 is a memory that is embeddedwithin the processor or within logic associated with bus 620 orwake-and-go array 622. Thread state storage 612 may comprise memorycells that are dedicated to storing thread state for sleeping threadsthat have a target address in wake-and-go array 622. In an alternativeembodiment, thread state storage 612 may be a hidden area of memory 332,for example. Thread state storage 612 may private in that it cannot beaddressed by the operating system or work threads.

FIGS. 7A and 7B are flowcharts illustrating operation of a wake-and-gomechanism in accordance with the illustrative embodiments. It will beunderstood that each block of the flowchart illustrations, andcombinations of blocks in the flowchart illustrations, can beimplemented by computer program instructions. These computer programinstructions may be provided to a processor or other programmable dataprocessing apparatus to produce a machine, such that the instructionswhich execute on the processor or other programmable data processingapparatus create means for implementing the functions specified in theflowchart block or blocks. These computer program instructions may alsobe stored in a computer-readable memory or storage medium that candirect a processor or other programmable data processing apparatus tofunction in a particular manner, such that the instructions stored inthe computer-readable memory or storage medium produce an article ofmanufacture including instruction means which implement the functionsspecified in the flowchart block or blocks.

Accordingly, blocks of the flowchart illustrations support combinationsof means for performing the specified functions, combinations of stepsfor performing the specified functions and program instruction means forperforming the specified functions. It will also be understood that eachblock of the flowchart illustrations, and combinations of blocks in theflowchart illustrations, can be implemented by special purposehardware-based computer systems which perform the specified functions orsteps, or by combinations of special purpose hardware and computerinstructions.

Furthermore, the flowcharts are provided to demonstrate the operationsperformed within the illustrative embodiments. The flowcharts are notmeant to state or imply limitations with regard to the specificoperations or, more particularly, the order of the operations. Theoperations of the flowcharts may be modified to suit a particularimplementation without departing from the spirit and scope of thepresent invention.

With reference now to FIG. 7A, operation begins when a thread firstinitializes or when a thread wakes after sleeping. The operating systemstarts a thread (block 702) by initializing the thread and placing thethread in the run queue for a processor. The thread then performs work(block 704). The operating system determines whether the thread hascompleted (block 706). If the thread completes, then operation ends.

If the end of the thread is not reached in block 706, the processordetermines whether the next instruction updates the wake-and-go array(block 708). An instruction to update the wake-and-go array may be aspecialized processor instruction, an operating system call, a call to abackground sleeper thread, or a call to an application programminginterface. If the next instruction does not update the wake-and-goarray, operation returns to block 704 to perform more work.

If the next instruction does update the wake-and-go array in block 708,the processor updates the array with a target address associated with anevent for which the thread is waiting (block 710). The update to thewake-and-go array may be made by the thread through a specializedprocessor instruction, the operating system, or a background sleeperthread. Next, the operating system then determines whether to put thethread to sleep (block 712). The operating system may keep the threadactive in the processor if the processor is underutilized, for instance;however, the operating system may put the thread to sleep if there areother threads waiting to be run on the processor. If the operatingsystem determines that the thread is to remain active, operation returnsto block 704 to perform more work, in which case the thread may simplywait for the event.

In one exemplary embodiment, f the operating system determines that thethread is to be put to sleep in block 712, then the operating system orsome other software or hardware saves the state of the thread (block714) and puts the thread to sleep (block 716). Thereafter, operationproceeds to FIG. 7B where the wake-and-go mechanism monitors for anevent. In one exemplary embodiment, software may save the state of thethread in thread state storage. The thread is then put to sleep.

In an alternative embodiment, if the operating system determines thatthe thread is to be put to sleep in block 712, then the operating systemor some other software or hardware saves the state of the thread (block714) in the hardware private array and puts the thread to sleep (block716). Thereafter, operation proceeds to FIG. 7B where the wake-and-gomechanism monitors for an event.

With reference now to FIG. 7B, the wake-and-go mechanism, which mayinclude a wake-and-go array, such as a content addressable memory, andassociated logic, snoops for a kill from the symmetric multiprocessing(SMP) fabric (block 718). A kill occurs when a transaction appears onthe SMP fabric that modifies the target address associated with theevent for which a thread is waiting. The wake-and-go mechanism thenperforms a compare (block 720) and determines whether the value beingwritten to the target address represents the event for which the threadis waiting (block 722). If the kill corresponds to the event for whichthe thread is waiting, then the operating system updates the array(block 724) to remove the target address from the wake-and-go array.Thereafter, operation returns to block 702 in FIG. 7A where theoperating system restarts the thread.

In one exemplary embodiment, when the wake-and-go mechanism snoops akill at a target address, the wake-and-go mechanism may generate anexception. The processor sees the exception and performs a trap, whichresults in a switch to kernel mode, wherein the operating system mayperform some action before returning control to the originating process.In this case, the trap results in other software to reload the threadfrom the thread state storage and to continue processing of the activethreads on the processor in block 702.

In one exemplary embodiment, when the wake-and-go mechanism snoops akill at a target address, software or hardware reloads the thread fromthe hardware private array and the processor continues processing theactive threads on the processor in block 702.

If the kill does not correspond to the event for which the thread iswaiting in block 722, then operation returns to block 718 to snoop akill from the SMP fabric. In FIG. 7B, the wake-and-go mechanism may be acombination of logic associated with the wake-and-go array, such as aCAM, and software within the operating system, software within abackground sleeper thread, or other hardware.

In an alternative embodiment, the wake-and-go mechanism may be acombination of logic associated with the wake-and-go array and softwarewithin the thread itself. In such an embodiment, the thread will wakeevery time there is a kill to the target address. The thread itself maythen perform a compare operation to determine whether to perform morework or to go back to sleep. If the thread decides to go back to sleep,it may again save the state of the thread. The over head for waking thethread every time there is a kill to the target address will likely bemuch less than polling or event handlers.

Prioritization of Threads

FIGS. 8A and 8B are flowcharts illustrating operation of a wake-and-gomechanism with prioritization of threads in accordance with theillustrative embodiments. Operation begins when a thread firstinitializes or when a thread wakes after sleeping. The operating systemstarts a thread (block 802) by initializing the thread and placing thethread in the run queue for a processor. The thread then performs work(block 804). The operating system determines whether the thread hascompleted (block 806). If the thread completes, then operation ends.

If the end of the thread is not reached in block 806, the processordetermines whether the next instruction updates the wake-and-go array(block 808). An instruction to update the wake-and-go array may be aspecialized processor instruction, an operating system call, a call to abackground sleeper thread, or a call to an application programminginterface. If the next instruction does not update the wake-and-goarray, operation returns to block 804 to perform more work.

If the next instruction does update the wake-and-go array in block 808,the wake-and-go mechanism determines whether there is sufficient spacefor the thread state in the hardware private array (block 810). If thereis sufficient space available, the wake-and-go mechanism allocates spacefor the thread state in the hardware private array (block 812). Thisallocation may simply comprise reserving the requisite space for thethread space, which may be about 1000 bytes, for example. If thehardware private array is reserved portion of system memory, thenallocating space may comprise requesting more system memory to bereserved for the hardware private array. Then, the wake-and-go mechanismsaves the state of the thread in the hardware private array (block 814),updates the wake-and-go array with the target address and otherinformation, if any (block 816), and puts the thread to sleep (block818). Thereafter, operation proceeds to FIG. 8B where the wake-and-gomechanism monitors for an event.

If there is insufficient space for the thread state available in thehardware private array in block 810, then the wake-and-go mechanismdetermines whether there is at least one lower priority thread in thehardware private array or wake-and-go array (block 820). As describedabove, each thread may have an associated priority parameter that isstored in the hardware private array or wake-and-go array.Alternatively, priority may be determined by other factors, such asstaleness. If there is at least one lower priority thread in thehardware private array, the wake-and-go mechanism removes the lowerpriority thread from the hardware private array and wake-and-go array(block 822) and converts the lower priority thread to a flee model(block 824). Thereafter, operation proceeds to block 814 to save thestate of the new thread, update the wake-and-go array, and put thethread to sleep.

If there is not a lower priority thread in the hardware private array inblock 820, the wake-and-go mechanism converts the new thread to a fleemodel (block 826). Thereafter, operation proceeds to block 818 to putthe thread to sleep.

With reference now to FIG. 8B, the wake-and-go mechanism, which mayinclude a wake-and-go array, such as a content addressable memory, andassociated logic, snoops for a kill from the symmetric multiprocessing(SMP) fabric (block 826). A kill occurs when a transaction appears onthe SMP fabric that modifies the target address associated with theevent for which a thread is waiting. The wake-and-go mechanism thenperforms a compare (block 828) and determines whether the value beingwritten to the target address represents the event for which the threadis waiting (block 830). If the kill corresponds to the event for whichthe thread is waiting, then the operating system updates the wake-and-goarray (block 832) to remove the target address from the wake-and-goarray. Then, the wake-and-go mechanism reloads the thread from thehardware private array (block 834). Thereafter, operation returns toblock 802 in FIG. 8A where the operating system restarts the thread.

In one exemplary embodiment, when the wake-and-go mechanism snoops akill at a target address, software or hardware reloads the thread fromthe hardware private array and the processor continues processing theactive threads on the processor in block 802.

If the kill does not correspond to the event for which the thread iswaiting in block 830, then operation returns to block 826 to snoop akill from the SMP fabric. In FIG. 8B, the wake-and-go mechanism may be acombination of logic associated with the wake-and-go array, such as aCAM, and software within the operating system, software within abackground sleeper thread, or other hardware.

Dynamic Allocation in Hardware Private Array

FIGS. 9A and 9B are flowcharts illustrating operation of a wake-and-gomechanism with dynamic allocation in a hardware private array inaccordance with the illustrative embodiments. Operation begins when athread first initializes or when a thread wakes after sleeping. Thewake-and-go mechanism allocates space for thread state information inthe hardware private array (block 902). The operating system starts athread (block 904) by initializing the thread and placing the thread inthe run queue for a processor. The wake-and-go mechanism may alsoallocate space in the wake-and-go array. The thread then performs work(block 906). The operating system determines whether the thread hascompleted (block 908). If the thread completes, then the wake-and-gomechanism de-allocates the space corresponding to the thread stateinformation for the thread (block 910), and operation ends.

If the end of the thread is not reached in block 908, the processordetermines whether the next instruction updates the wake-and-go array(block 912). An instruction to update the wake-and-go array may be aspecialized processor instruction, an operating system call, a call to abackground sleeper thread, or a call to an application programminginterface. If the next instruction does not update the wake-and-goarray, operation returns to block 906 to perform more work.

If the next instruction does update the wake-and-go array in block 912,the wake-and-go mechanism updates the wake-and-go array with a targetaddress associated with an event for which the thread is waiting (block914). The update to the wake-and-go array may be made by the threadthrough a specialized processor instruction, the operating system, or abackground sleeper thread. Next, the operating system then determineswhether to put the thread to sleep (block 916). The operating system maykeep the thread active in the processor if the processor isunderutilized, for instance; however, the operating system may put thethread to sleep if there are other threads waiting to be run on theprocessor. If the operating system determines that the thread is toremain active, operation returns to block 906 to perform more work, inwhich case the thread may simply wait for the event.

If the operating system determines that the thread is to be put to sleepin block 916, then the operating system or some other software orhardware saves the state of the thread (block 918) in the hardwareprivate array and puts the thread to sleep (block 920). Thereafter,operation proceeds to FIG. 9B where the wake-and-go mechanism monitorsfor an event.

With reference now to FIG. 9B, the wake-and-go mechanism, which mayinclude a wake-and-go array, such as a content addressable memory, andassociated logic, snoops for a kill from the symmetric multiprocessing(SMP) fabric (block 922). A kill occurs when a transaction appears onthe SMP fabric that modifies the target address associated with theevent for which a thread is waiting. The wake-and-go mechanism thenperforms a compare (block 924) and determines whether the value beingwritten to the target address represents the event for which the threadis waiting (block 926). If the kill corresponds to the event for whichthe thread is waiting, then the operating system updates the wake-and-goarray (block 928) to remove the target address from the wake-and-goarray. The wake-and-go mechanism then reloads the thread state from thehardware private array (block 930). Thereafter, operation returns toblock 904 in FIG. 9A where the operating system restarts the thread.

If the kill does not correspond to the event for which the thread iswaiting in block 922, then operation returns to block 922 to snoop akill from the SMP fabric. In FIG. 9B, the wake-and-go mechanism may be acombination of logic associated with the wake-and-go array, such as aCAM, and software within the operating system, software within abackground sleeper thread, or other hardware.

Hardware Wake-and-Go Mechanism

FIG. 10 is a block diagram of a hardware wake-and-go mechanism in a dataprocessing system in accordance with an illustrative embodiment. Threads1002, 1004, 1006 run on processor 1000. Threads 1002, 1004, 1006 makecalls to operating system 1010 to communicate with each other, memory1032 via bus 1020, or other devices within the data processing system.While the data processing system in FIG. 10 shows one processor, moreprocessors may be present depending upon the implementation where eachprocessor has a separate wake-and-go array or one wake-and-go arraystores target addresses for threads for multiple processors.

Wake-and-go mechanism 1008 is a hardware implementation within processor1000. In an alternative embodiment, hardware wake-and-go mechanism 1008may be logic associated with wake-and-go array 1022 attached to bus 1020or a separate, dedicated wake-and-go engine as described in furtherdetail below.

In accordance with the illustrative embodiment, hardware wake-and-gomechanism 1008 is provided within processor 1000 and wake-and-go array1022 is attached to the SMP fabric. The SMP fabric is a communicationmedium through which processors communicate. The SMP fabric may comprisea single SMP bus or a system of busses, for example. In the depictedexample, the SMP fabric comprises bus 1020. A thread, such as thread1002, for example, may include instructions that indicate that thethread is waiting for an event. The event may be an asynchronous event,which is an event that happens independently in time with respect toexecution of the thread in the data processing system. For example, anasynchronous event may be a temperature value reaching a particularthreshold, a stock price falling below a given threshold, or the like.Alternatively, the event may be related in some way to execution of thethread. For example, the event may be obtaining a lock for exclusiveaccess to a database record or the like.

Processor 1000 may pre-fetch instructions from storage (not shown) tomemory 1032. These instructions may comprise a get-and-compare sequence,for example. Wake-and-go mechanism 1008 within processor 1000 mayexamine the instruction stream as it is being pre-fetched and recognizethe get-and-compare sequence as a programming idiom that indicates thatthread 1002 is waiting for data at a particular target address. Aprogramming idiom is a sequence of programming instructions that occursoften and is recognizable as a sequence of instructions. In thisexample, an instruction sequence that includes load (LD), compare (CMP),and branch (BC) commands represents a programming idiom that indicatesthat the thread is waiting for data to be written to a particular targetaddress. In this case, wake-and-go mechanism 1008 recognizes such aprogramming idiom and may store the target address in wake-and-go array1022, where the event the thread is waiting for is associated with thetarget address. After updating wake-and-go array 1022 with the targetaddress, wake-and-go mechanism 1008 may put thread 1002 to sleep.

Wake-and-go mechanism 1008 also may save the state of thread 1002 inthread state storage 1034, which may be allocated from memory 1032 ormay be a hardware private array within the processor (not shown) orpervasive logic (not shown). When a thread is put to sleep, i.e.,removed from the run queue of a processor, the operating system muststore sufficient information on its operating state such that when thethread is again scheduled to run on the processor, the thread can resumeoperation from an identical position. This state information is sometimereferred to as the thread's “context.” The state information mayinclude, for example, address space, stack space, virtual address space,program counter, instruction register, program status word, and thelike.

If a transaction appears on bus 1020 that modifies a value at an addressin wake-and-go array 1022, then wake-and-go mechanism 1008 may wakethread 1002. Wake-and-go mechanism 1008 may wake thread 1002 byrecovering the state of thread 1002 from thread state storage 1034.Thread 1002 may then determine whether the transaction corresponds tothe event for which the thread was waiting by performing aget-and-compare operation, for instance. If the transaction is the eventfor which the thread was waiting, then thread 1002 will perform work.However, if the transaction is not the event, then thread 1002 will goback to sleep. Thus, thread 1002 only performs a get-and-compareoperation if there is a transaction that modifies the target address.

Alternatively, operating system 1010 or a background sleeper thread,such as thread 1004, may determine whether the transaction is the eventfor which the thread was waiting. Before being put to sleep, thread 1002may update a data structure in the operating system or backgroundsleeper thread with a value for which it is waiting.

In one exemplary embodiment, wake-and-go array 1022 may be a contentaddressable memory (CAM). A CAM is a special type of computer memoryoften used in very high speed searching applications. A CAM is alsoknown as associative memory, associative storage, or associative array,although the last term is more often used for a programming datastructure. Unlike a random access memory (RAM) in which the usersupplies a memory address and the RAM returns the data value stored atthat address, a CAM is designed such that the user supplies a data valueand the CAM searches its entire memory to see if that data value isstored within the CAM. If the data value is found, the CAM returns alist of one or more storage addresses where the data value was found. Insome architectures, a CAM may return the data value or other associatedpieces of data. Thus, a CAM may be considered the hardware embodiment ofwhat in software terms would be called an associative array.

Thus, in an exemplary embodiment, wake-and-go array 1022 may comprise aCAM and associated logic that will be triggered if a transaction appearson bus 1020 that modifies an address stored in the CAM. A transactionthat modifies a value at a target address may be referred to as a“kill”; thus, wake-and-go array 1022 may be said to be “snooping kills.”In this exemplary embodiment, the data values stored in the CAM are thetarget addresses at which threads are waiting for something to bewritten. The address at which a data value, a given target address, isstored is referred to herein as the storage address. Each storageaddress may refer to a thread that is asleep and waiting for an event.Wake-and-go array 1022 may store multiple instances of the same targetaddress, each instance being associated with a different thread waitingfor an event at that target address. Thus, when wake-and-go array 1022snoops a kill at a given target address, wake-and-go array 1022 mayreturn one or more storage addresses that are associated with one ormore sleeping threads.

FIGS. 11A and 11B illustrate a series of instructions that are aprogramming idiom for wake-and-go in accordance with an illustrativeembodiment. With reference to FIG. 11A, the instruction sequenceincludes load (LD), compare (CMP), and branch (BC) commands thatrepresent a programming idiom that indicate that the thread is waitingfor data to be written to a particular target address. The load command(LD) loads a data value to general purpose register GPR D from theaddress in general purpose register GPR A. The compare command (CMP)then compares the value loaded into general purpose register GPR D witha value already stored in general purpose register GPR E. If the comparecommand results in a match, then the branch command (BC) branches toinstruction address IA.

The wake-and-go mechanism may recognize the poll operation idiom. Whenthe wake-and-go mechanism recognizes such a programming idiom, thewake-and-go mechanism may store the target address from GPR A in thewake-and-go array, where the event the thread is waiting for isassociated with the target address. After updating the wake-and-go arraywith the target address, the wake-and-go mechanism may put the thread tosleep.

With reference now to FIG. 11B, thread 1110 may have a plurality ofprogramming idioms. The wake-and-go mechanism may look ahead withinthread 1110 and load wake-and-go array 1122 with the target address andother information, if any. Therefore, when thread 1110 reaches eachprogramming idiom while executing, the wake-and-go array 1122 willalready be loaded with the target address, and thread 1110 may simply goto sleep until wake-and-go array snoops the target address on the SMPfabric.

The wake-and-go mechanism may perform a look-ahead polling operation foreach programming idiom. In the depicted example, idioms A, B, C, and Dfail. In those cases, the wake-and-go mechanism may update wake-and-goarray 1122. In this example, idiom E passes; therefore, there is no needto update wake-and-go array 1122, because there is no need to put thethread to sleep when idiom E executes.

In one exemplary embodiment, the wake-and-go mechanism may updatewake-and-go array 1122 only if all of the look-ahead polling operationsfail. If at least one look-ahead polling operation passes, then thewake-and-go mechanism may consider each idiom as it occurs duringexecution.

FIGS. 12A and 12B are block diagrams illustrating operation of ahardware wake-and-go mechanism in accordance with an illustrativeembodiment. With particular reference to FIG. 12A, thread 1210 runs in aprocessor (not shown) and performs some work. Thread 1210 executes aseries of instructions that are a programming idiom for wake-and-go. Thewake-and-go mechanism may recognize the poll operation idiom. When thewake-and-go mechanism recognizes such a programming idiom, thewake-and-go mechanism may store the target address A₂ in wake-and-goarray 1222, where the event the thread is waiting for is associated withthe target address, and stores thread state information for thread 1210in thread state storage 1212. After updating wake-and-go array 1222 withthe target address A₂, the wake-and-go mechanism may put the thread 1210to sleep.

When a transaction appears on SMP fabric 1220 with an address thatmatches the target address A₂, array 1222 returns the storage addressthat is associated with thread 1210. The wake-and-go mechanism thenwakes thread 1210 by retrieving the thread state information from threadstate storage 1212 and placing the thread in the run queue for theprocessor. Thread 1210 may then perform a compare-and-branch operationto determine whether the value written to the target address representsthe event for which thread 1210 is waiting. In the depicted example, thevalue written to the target address does not represent the event forwhich thread 1210 is waiting; therefore, thread 1210 goes back to sleep.

Turning to FIG. 12B, thread 1210 runs in a processor (not shown) andperforms some work. Thread 1210 executes a series of instructions thatare a programming idiom for wake-and-go. The wake-and-go mechanism mayrecognize the poll operation idiom. When the wake-and-go mechanismrecognizes such a programming idiom, the wake-and-go mechanism may storethe target address A₂ in wake-and-go array 1222, where the event thethread is waiting for is associated with the target address, and storesthread state information for thread 1210 in thread state storage 1212.After updating wake-and-go array 1222 with the target address A₂, thewake-and-go mechanism may put the thread 1210 to sleep.

When a transaction appears on SMP fabric 1220 with an address thatmatches the target address A₂, array 1222 returns the storage addressthat is associated with thread 1210. The wake-and-go mechanism thenwakes thread 1210 by retrieving the thread state information from threadstate storage 1212 and placing the thread in the run queue for theprocessor. Thread 1210 may then perform a compare-and-branch operationto determine whether the value written to the target address representsthe event for which thread 1210 is waiting. In the depicted example, thevalue written to the target address does represent the event for whichthread 1210 is waiting; therefore, thread 1210 updates the array toremove the target address from array 1222, and performs more work.

FIGS. 13A and 13B are flowcharts illustrating operation of a hardwarewake-and-go mechanism in accordance with the illustrative embodiments.Operation begins when a thread first initializes or when a thread wakesafter sleeping. The operating system starts a thread (block 1302) byinitializing the thread and placing the thread in the run queue for aprocessor. The thread then performs work (block 1304). The operatingsystem determines whether the thread has completed (block 1306). If thethread completes, then operation ends.

If the end of the thread is not reached in block 1306, the processordetermines whether the next instructions comprise a wake-and-go idiom,such as a polling operation, for example (block 1308). A wake-and-goidiom may comprise a series of instructions, such as a load, compare,and branch sequence, for example. If the next instructions doe notcomprise a wake-and-go idiom, the wake-and-go mechanism returns to block1304 to perform more work.

If the next instructions do comprise a wake-and-go idiom in block 1308,the wake-and-go mechanism determines whether to put the thread to sleep(block 1310). The wake-and-go mechanism may keep the thread active inthe processor if the processor is underutilized, for instance; however,the wake-and-go mechanism may put the thread to sleep if there are otherthreads waiting to be run on the processor. If the wake-and-go mechanismdetermines that the thread is to remain active, operation returns toblock 1304 to perform more work, in which case the thread may simplywait for the event.

If the wake-and-go mechanism determines that the thread is to be put tosleep in block 1310, then the wake-and-go mechanism updates the arraywith a target address associated with an event for which the thread iswaiting (block 1312). The update to the wake-and-go array may be made bythe thread through a specialized processor instruction, the operatingsystem, or a background sleeper thread. Next, the wake-and-go mechanismthen saves the state of the thread (block 1314) and puts the thread tosleep (block 1316). Thereafter, operation proceeds to FIG. 13B where thewake-and-go mechanism monitors for an event.

With reference now to FIG. 13B, the wake-and-go mechanism, which mayinclude a wake-and-go array, such as a content addressable memory, andassociated logic, snoops for a kill from the symmetric multiprocessing(SMP) fabric (block 1318). A kill occurs when a transaction appears onthe SMP fabric that modifies the target address associated with theevent for which a thread is waiting. The wake-and-go mechanism, theoperating system, the thread, or other software then performs a compare(block 1320) and determines whether the value being written to thetarget address represents the event for which the thread is waiting(block 1322). If the kill corresponds to the event for which the threadis waiting, then the wake-and-go mechanism updates the array (block1324) to remove the target address from the wake-and-go array.Thereafter, operation returns to block 1302 in FIG. 13A where theoperating system restarts the thread.

If the kill does not correspond to the event for which the thread iswaiting in block 1322, then operation returns to block 1318 to snoop akill from the SMP fabric. In FIG. 13B, the wake-and-go mechanism may bea combination of hardware within the processor, logic associated withthe wake-and-go array, which may be a CAM as described above, andsoftware within the operating system, software within a backgroundsleeper thread. In other embodiments, the wake-and-go mechanism may beother software or hardware, such as a dedicated wake-and-go engine, asdescribed in further detail below.

Look-Ahead Polling

FIGS. 14A and 14B are block diagrams illustrating operation of awake-and-go engine with look-ahead in accordance with an illustrativeembodiment. With particular reference to FIG. 14A, thread 1410 runs in aprocessor (not shown) and performs some work. Thread 1410 executes aseries of instructions that are a programming idiom for wake-and-go. Thewake-and-go mechanism may recognize the poll operation idiom. When thewake-and-go mechanism recognizes such a programming idiom, thewake-and-go mechanism may store the target address A₂ in wake-and-goarray 1422, where the event the thread is waiting for is associated withthe target address, and stores thread state information for thread 1410in thread state storage 1412. After updating wake-and-go array 1422 withthe target address A₂, the wake-and-go mechanism may put the thread 1410to sleep.

When a transaction appears on SMP fabric 1420 with an address thatmatches the target address A₂, array 1422 returns the storage addressthat is associated with thread 1410. The wake-and-go mechanism thenwakes thread 1410 by retrieving the thread state information from threadstate storage 1412 and placing the thread in the run queue for theprocessor. Thread 1410 may then perform a compare-and-branch operationto determine whether the value written to the target address representsthe event for which thread 1410 is waiting. In the depicted example, thevalue written to the target address does not represent the event forwhich thread 1410 is waiting; therefore, thread 1410 goes back to sleep.

Turning to FIG. 14B, thread 1410 runs in a processor (not shown) andperforms some work. Thread 1410 executes a series of instructions thatare a programming idiom for wake-and-go. The wake-and-go mechanism mayrecognize the poll operation idiom. When the wake-and-go mechanismrecognizes such a programming idiom, the wake-and-go mechanism may storethe target address A₂ in wake-and-go array 1422, where the event thethread is waiting for is associated with the target address, and storesthread state information for thread 1410 in thread state storage 1412.After updating wake-and-go array 1422 with the target address A₂, thewake-and-go mechanism may put the thread 1410 to sleep.

When a transaction appears on SMP fabric 1420 with an address thatmatches the target address A₂, array 1422 returns the storage addressthat is associated with thread 1410. The wake-and-go mechanism thenwakes thread 1410 by retrieving the thread state information from threadstate storage 1412 and placing the thread in the run queue for theprocessor. Thread 1410 may then perform a compare-and-branch operationto determine whether the value written to the target address representsthe event for which thread 1410 is waiting. In the depicted example, thevalue written to the target address does represent the event for whichthread 1410 is waiting; therefore, thread 1410 updates the array toremove the target address from array 1422, and performs more work.

FIG. 15 is a flowchart illustrating a look-ahead polling operation of awake-and-go look-ahead engine in accordance with an illustrativeembodiment. Operation begins, and the wake-and-go look-ahead engineexamines the thread for programming idioms (block 1502). Then, thewake-and-go look-ahead engine determines whether it has reached the endof the thread (block 1504). If the wake-and-go look-ahead engine hasreached the end of the thread, operation ends.

If the wake-and-go look-ahead engine has not reached the end of thethread in block 1504, the wake-and-go look-ahead engine determineswhether the thread comprises at least one wake-and-go programming idiomthat indicates that the thread is waiting for a data value to be writtento a particular target address (block 1506). If the thread does notcomprise a wake-and-go programming idiom, operation ends.

If the thread does comprise at least one wake-and-go programming idiomin block 1506, then the wake-and-go look-ahead engine performs load andcompare operations for the at least one wake-and-go programming idiom(block 1508). Thereafter, the wake-and-go look-ahead engine determineswhether all of the load and compare operations fail (block 1510). If allof the look-ahead polling operations fail, then the wake-and-golook-ahead engine updates the wake-and-go array for the at least oneprogramming idiom (block 1512), and operation ends. If at least onelook-ahead polling operation succeeds, then operation ends withoutupdating the wake-and-go array. In an alternative embodiment, thelook-ahead engine may set up the wake-and-go array without performinglook-ahead polling.

Speculative Execution

FIG. 16 is a block diagram illustrating operation of a wake-and-gomechanism with speculative execution in accordance with an illustrativeembodiment. Thread 1610 runs in a processor (not shown) and performssome work. Thread 1610 also includes a series of instructions that are aprogramming idiom for wake-and-go (idiom A), along with idioms B, C, D,and E from FIG. 11B.

Look-ahead wake-and-go engine 1620 analyzes the instructions in thread410 ahead of execution. Look-ahead wake-and-go engine 1620 may recognizethe poll operation idioms and perform look-ahead polling operations foreach idiom. If the look-ahead polling operation fails, the look-aheadwake-and-go engine 1620 populates wake-and-go array 1622 with the targetaddress. In the depicted example from FIG. 11B, idioms A-D fail;therefore, look-ahead wake-and-go engine 1620 populates wake-and-goarray 1622 with addresses A₁-A₄, which are the target addresses foridioms A-D.

If a look-ahead polling operation succeeds, look-ahead wake-and-goengine 1620 may record an instruction address for the correspondingidiom so that the wake-and-go mechanism may have thread 1610 performspeculative execution at a time when thread 1610 is waiting for anevent. During execution, when the wake-and-go mechanism recognizes aprogramming idiom, the wake-and-go mechanism may store the thread statein thread state storage 1612. Instead of putting thread 1610 to sleep,the wake-and-go mechanism may perform speculative execution.

When a transaction appears on SMP fabric 1620 with an address thatmatches the target address A₁, array 1622 returns the storage addressthat is associated with thread 1610 to the wake-and-go mechanism. Thewake-and-go mechanism then returns thread 1610 to the state at whichidiom A was encountered by retrieving the thread state information fromthread state storage 1612. Thread 1610 may then continue work from thepoint of idiom A.

FIG. 17 is a flowchart illustrating operation of a look-aheadwake-and-go mechanism with speculative execution in accordance with anillustrative embodiment. Operation begins, and the wake-and-golook-ahead engine examines the thread for programming idioms (block1702). Then, the wake-and-go look-ahead engine determines whether it hasreached the end of the thread (block 1704). If the wake-and-golook-ahead engine has reached the end of the thread, operation ends.

If the wake-and-go look-ahead engine has not reached the end of thethread in block 1704, the wake-and-go look-ahead engine determineswhether next sequence of instructions comprises a wake-and-goprogramming idiom that indicates that the thread is waiting for a datavalue to be written to a particular target address (block 1706). If thenext sequence of instructions does not comprise a wake-and-goprogramming idiom, operation returns to block 502 to examine the nextsequence of instructions in the thread. A wake-and-go programming idiommay comprise a polling idiom, as described with reference to FIG. 11A.

If the next sequence of instructions does comprise a wake-and-goprogramming idiom in block 1706, then the wake-and-go look-ahead engineperforms load and compare operations for the wake-and-go programmingidiom (block 1708). Thereafter, the wake-and-go look-ahead enginedetermines whether the load and compare operation passes (block 1710).If the look-ahead polling operation fails, then the wake-and-golook-ahead engine updates the wake-and-go array for the programmingidiom (block 1712), and operation returns to block 1702 to examine thenext sequence of instructions in the thread. If the look-ahead pollingoperation passes, then the look-ahead wake-and-go engine records aninstruction address for the successful programming idiom to be used forspeculative execution later (block 1714). Thereafter, operation ends.

FIGS. 18A and 18B are flowcharts illustrating operation of a wake-and-gomechanism with speculative execution during execution of a thread inaccordance with an illustrative embodiment. With reference now to FIG.18A, operation begins when a thread first initializes or when a threadwakes after sleeping. The operating system starts a thread (block 1802)by initializing the thread and placing the thread in the run queue for aprocessor. The thread then performs work (block 1804). The operatingsystem determines whether the thread has completed (block 1806). If thethread completes, then operation ends.

If the end of the thread is not reached in block 1806, the processordetermines whether the next instructions comprise a wake-and-go idiom,such as a polling operation, for example (block 1808). A wake-and-goidiom may comprise a series of instructions, such as a load, compare,and branch sequence, for example. If the next instructions do notcomprise a wake-and-go idiom, the wake-and-go mechanism returns to block1804 to perform more work.

If the next instructions do comprise a wake-and-go idiom in block 1808,the wake-and-go mechanism saves the state of the thread (block 1810).Then, the wake-and-go mechanism determines whether to performspeculative execution (block 1812). The wake-and-go mechanism may makethis determination by determining whether the look-ahead wake-and-goengine previously performed a successful look-ahead polling operationand recorded an instruction address.

If the wake-and-go mechanism determines that the processor cannotperform speculative execution, the wake-and-go mechanism puts the threadto sleep. Thereafter, operation proceeds to FIG. 18B where thewake-and-go mechanism monitors for an event.

If the wake-and-go mechanism determines that the processor can performspeculative execution from a successful polling idiom, the wake-and-gomechanism begins performing speculative execution from the successfullypolled idiom (block 616). Thereafter, operation proceeds to FIG. 18Bwhere the wake-and-go mechanism monitors for an event.

With reference now to FIG. 18B, the wake-and-go mechanism, which mayinclude a wake-and-go array, such as a content addressable memory, andassociated logic, snoops for a kill from the symmetric multiprocessing(SMP) fabric (block 1818). A kill occurs when a transaction appears onthe SMP fabric that modifies the target address associated with theevent for which a thread is waiting. The wake-and-go mechanism, theoperating system, the thread, or other software then performs a compare(block 1820) and determines whether the value being written to thetarget address represents the event for which the thread is waiting(block 1822). If the kill corresponds to the event for which the threadis waiting, then the wake-and-go mechanism updates the array (block1824) to remove the target address from the wake-and-go array.Thereafter, operation returns to block 1804 in FIG. 18A where theprocessor performs more work.

If the kill does not correspond to the event for which the thread iswaiting in block 1822, then operation returns to block 1818 to snoop akill from the SMP fabric. In FIG. 18B, the wake-and-go mechanism may bea combination of hardware within the processor, logic associated withthe wake-and-go array, such as a CAM, and software within the operatingsystem, software within a background sleeper thread, or other hardware.

Data Monitoring

Returning to FIG. 10, the instructions may comprise a get-and-comparesequence, for example. Wake-and-go mechanism 1008 within processor 1000may recognize the get-and-compare sequence as a programming idiom thatindicates that thread 1002 is waiting for data at a particular targetaddress. When wake-and-go mechanism 1008 recognizes such a programmingidiom, wake-and-go mechanism 1008 may store the target address, the datathread 1002 is waiting for, and a comparison type in wake-and-go array1022, where the event the thread is waiting for is associated with thetarget address. After updating wake-and-go array 1022 with the targetaddress, wake-and-go mechanism 1008 may put thread 1002 to sleep.

The get-and-compare sequence may load a data value from a targetaddress, perform a compare operation based on an expected data value,and branch if the compare operation matches. Thus, the get-and-comparesequence had three basic elements: an address, an expected data value,and a comparison type. The comparison type may be, for example, equal to(=), less than (<), greater than (>), less than or equal to (≦), orgreater than or equal to (≧). Thus, wake-and-go mechanism 1008 may storethe address, data value, and comparison value in wake-and-go array 1022.

Thread 1002 may alternatively include specialized processorinstructions, operating system calls, or application programminginterface (API) calls that instruct wake-and-go mechanism 1008 topopulate wake-and-go array 1022 with a given address, data value, andcomparison type.

Wake-and-go mechanism 1008 also may save the state of thread 1002 inthread state storage 1034, which may be allocated from memory 1032 ormay be a hardware private array within the processor (not shown) orpervasive logic (not shown). When a thread is put to sleep, i.e.,removed from the run queue of a processor, the operating system muststore sufficient information on its operating state such that when thethread is again scheduled to run on the processor, the thread can resumeoperation from an identical position. This state information is sometimereferred to as the thread's “context.” The state information mayinclude, for example, address space, stack space, virtual address space,program counter, instruction register, program status word, and thelike.

If a transaction appears on bus 1020 that modifies a value at an addresswhere the value satisfies the comparison type in wake-and-go array 1022,then wake-and-go mechanism 1008 may wake thread 1002. Wake-and-go array1022 may have associated logic that recognizes the target address on bus1020 and performs the comparison based on the value being written, theexpected value stored in wake-and-go array 1022, and the comparison typestored in wake-and-go array 1022. Wake-and-go mechanism 1008 may wakethread 1002 by recovering the state of thread 1002 from thread statestorage 1034. Thus, thread 1002 only wakes if there is a transactionthat modifies the target address with a value that satisfies thecomparison type and expected value.

Thus, in an exemplary embodiment, wake-and-go array 1022 may comprise aCAM and associated logic that will be triggered if a transaction appearson bus 1020 that modifies an address stored in the CAM. A transactionthat modifies a value at a target address may be referred to as a“kill”; thus, wake-and-go array 1022 may be said to be “snooping kills.”In this exemplary embodiment, the data values stored in the CAM are thetarget addresses at which threads are waiting for something to bewritten, an expected value, and a comparison type. The address at whicha data value, a given target address, is stored is referred to herein asthe storage address.

Each storage address may refer to a thread that is asleep and waitingfor an event. Wake-and-go array 1022 may store multiple instances of thesame target address, each instance being associated with a differentthread waiting for an event at that target address. The expected valuesand comparison types may be different. Thus, when wake-and-go array 1022snoops a kill at a given target address, wake-and-go array 1022 mayreturn one or more storage addresses that are associated with one ormore sleeping threads. When wake-and-go array 1022 snoops a kill at thegiven target address, wake-and-go array 1022 may also return theexpected value and comparison type to associated logic that performs thecomparison. If the comparison matches, then the associated logic mayreturn a storage address to wake-and-go mechanism 1008 to wake thecorresponding thread.

FIG. 19 is a block diagram illustrating data monitoring in a multipleprocessor system in accordance with an illustrative embodiment.Processors 1902-1908 connect to bus 1920. Each one of processors1902-1908 may have a wake-and-go mechanism, such as wake-and-gomechanism 1008 in FIG. 10, and a wake-and-go array, such as wake-and-goarray 1022 in FIG. 10. A device (not shown) may modify a data value at atarget address through input/output channel controller (HOC) 1912, whichtransmits the transaction on bus 1920 to memory controller 1914.

The wake-and-go array of each processor 1902-1908 snoops bus 1920. If atransaction appears on bus 1920 that modifies a value at an addresswhere the value satisfies the comparison type in a wake-and-go array,then the wake-and-go mechanism may wake a thread. Each wake-and-go arraymay have associated logic that recognizes the target address on bus 1920and performs the comparison based on the value being written, theexpected value stored in the wake-and-go array, and the comparison typestored in the wake-and-go array. Thus, the wake-and-go mechanism mayonly wake a thread if there is a transaction on bus 1920 that modifiesthe target address with a value that satisfies the comparison type andexpected value.

FIG. 20 is a block diagram illustrating operation of a wake-and-gomechanism in accordance with an illustrative embodiment. Thread 2010runs in a processor (not shown) and performs some work. Thread 2010executes a series of instructions that are a programming idiom forwake-and-go, a specialized processor instruction, an operating systemcall, or an application programming interface (API) call. Thewake-and-go mechanism may recognize the idiom, specialized processorinstruction, operating system call, or API call, hereinafter referred toas a “wake-and-go operation.” When the wake-and-go mechanism recognizessuch a wake-and-go operation, the wake-and-go mechanism may store thetarget address A₂, expected data value D₂, and comparison type T₂ inwake-and-go array 2022, and stores thread state information for thread2010 in thread state storage 2012. After updating wake-and-go array 2022with the target address A₂, expected data value D₂, and comparison typeT₂, the wake-and-go mechanism may put thread 2010 to sleep.

When a transaction appears on SMP fabric 2020 with an address thatmatches the target address A₂, logic associated with wake-and-go array2022 may perform a comparison based on the value being written, theexpected value D₂ and the comparison type T₂. If the comparison is amatch, then the logic associated with wake-and-go array 2022 returns thestorage address that is associated with thread 2010. The wake-and-gomechanism then wakes thread 2010 by retrieving the thread stateinformation from thread state storage 2012 and placing the thread in therun queue for the processor.

Parallel Lock Spinning

Returning to FIG. 10, the instructions may comprise a get-and-comparesequence, for example. In an illustrative embodiment, the instructionsmay comprise a sequence of instructions that indicate that thread 1002is spinning on a lock. A lock is a synchronization mechanism forenforcing limits on access to resources in an environment where thereare multiple threads of execution. Generally, when a thread attempts towrite to a resource, the thread may request a lock on the resource toobtain exclusive access. If another thread already has the lock, thethread may “spin” on the lock, which means repeatedly polling the locklocation until the lock is free. The instructions for spinning on thelock represent an example of a programming idiom.

Wake-and-go mechanism 1008 within processor 1000 may recognize thespinning on lock idiom that indicates that thread 1002 is spinning on alock. When wake-and-go mechanism 1008 recognizes such a programmingidiom, wake-and-go mechanism 1008 may store the target address inwake-and-go array 1022 with a flag to indicate that thread 1002 isspinning on a lock. After updating wake-and-go array 1022 with thetarget address and setting the lock flag, wake-and-go mechanism 1008 mayput thread 1002 to sleep. Thus, wake-and-go mechanism 1008 allowsseveral threads to be spinning on a lock at the same time without usingvaluable processor resources.

If a transaction appears on bus 1020 that modifies a value at an addressin wake-and-go array 1022, then wake-and-go mechanism 1008 may wakethread 1002. Wake-and-go mechanism 1008 may wake thread 1002 byrecovering the state of thread 1002 from thread state storage 1034.Thread 1002 may then determine whether the transaction corresponds tothe event for which the thread was waiting by performing aget-and-compare operation, for instance. If the lock bit is set inwake-and-go array 1022, then it is highly likely that the transaction isfreeing the lock, in which case, wake-and-go mechanism may automaticallywake thread 1002.

FIGS. 21A and 21B are block diagrams illustrating parallel lock spinningusing a wake-and-go mechanism in accordance with an illustrativeembodiment. With particular reference to FIG. 21A, thread 2110 runs in aprocessor (not shown) and performs some work. Thread 2110 executes aseries of instructions that are a programming idiom for spin on lock.The wake-and-go mechanism may recognize the spin on lock operationidiom. When the wake-and-go mechanism recognizes such a programmingidiom, the wake-and-go mechanism may store the target address A₁ inwake-and-go array 2122, set the lock bit 2124, and store thread stateinformation for thread 2110 in thread state storage 2112. After updatingwake-and-go array 2122 with the target address A₁, the wake-and-gomechanism may put the thread 2110 to sleep.

The processor may then run thread 2130, which performs some work. Thewake-and-go mechanism may recognize a spin on lock operation idiom,responsive to which the wake-and-go mechanism stores the target addressA₂ in wake-and-go array 2122, set the lock bit 2124, and store threadstate information for thread 2130 in thread state storage 2112. Afterupdating wake-and-go array 2122 with the target address A₂, thewake-and-go mechanism may put the thread 2130 to sleep.

Turning to FIG. 21B, thread 2140 runs in the processor and performs somework. When a transaction appears on SMP fabric 2120 with an address thatmatches the target address A₁, wake-and-go array 2122 returns thestorage address that is associated with thread 2110. The wake-and-gomechanism then wakes thread 2110 by retrieving the thread stateinformation from thread state storage 2112 and placing the thread in therun queue for the processor, because it is highly likely that thetransaction is freeing the lock. Thread 2110 may update array 2122 toremove the target address. In the depicted example, thread 2110 andthread 2140 run concurrently in the processor. Thus, thread 2110 andthread 2130, and any number of other threads, may be spinning on a lockat the same time. When a lock is freed, the processor may wake thethread, such as thread 2110 in the depicted example, and the remainingthreads may continue “spinning” on the lock without consuming anyprocessor resources.

FIGS. 22A and 22B are flowcharts illustrating parallel lock spinningusing a wake-and-go mechanism in accordance with the illustrativeembodiments. Operation begins when a thread first initializes or when athread wakes after sleeping. The operating system starts a thread (block2202) by initializing the thread and placing the thread in the run queuefor a processor. The thread then performs work (block 2204). Theoperating system determines whether the thread has completed (block2206). If the thread completes, then operation ends.

If the end of the thread is not reached in block 2206, the processordetermines whether the next instructions comprise a spin on lock idiom(block 2208). A spin on lock idiom may comprise a series ofinstructions, such as a load, compare, and branch sequence, for example.If the next instructions do not comprise a spin on lock idiom, thewake-and-go mechanism returns to block 2204 to perform more work.

If the next instructions do comprise a spin on lock idiom in block 2208,the wake-and-go mechanism updates the array with a target addressassociated with an event for which the thread is waiting (block 2210)and sets the lock bit in the wake-and-go array (block 2212). The updateto the wake-and-go array may be made by the thread through a specializedprocessor instruction, the operating system, or a background sleeperthread. Next, the wake-and-go mechanism saves the state of the thread(block 2214) and puts the thread to sleep (block 2216). Thereafter,operation proceeds to FIG. 22B where the wake-and-go mechanism monitorsfor an event.

With reference now to FIG. 22B, the wake-and-go mechanism, which mayinclude a wake-and-go array, such as a content addressable memory (CAM),and associated logic, snoops for a kill from the symmetricmultiprocessing (SMP) fabric (block 2218). A kill occurs when atransaction appears on the SMP fabric that modifies the target addressassociated with the event for which a thread is waiting. The wake-and-gomechanism determines whether the value being written to the targetaddress represents the event for which the thread is waiting (block2220). If the lock bit is set, then it is highly likely that the eventis merely freeing the lock. If the kill corresponds to the event forwhich the thread is waiting, then the wake-and-go mechanism updates thearray (block 2222) to remove the target address from the wake-and-goarray and reloads the thread state for the thread that was spinning onthe lock (block 2224). Thereafter, operation returns to block 2202 inFIG. 22A where the operating system restarts the thread.

If the kill does not correspond to the event for which the thread iswaiting in block 2220, then operation returns to block 2218 to snoop akill from the SMP fabric. In FIG. 22B, the wake-and-go mechanism may bea combination of hardware within the processor, logic associated withthe wake-and-go array, such as a CAM, and software within the operatingsystem, software within a background sleeper thread, or other hardware.

Central Repository for Wake-and-Go Engine

As stated above with reference to FIG. 10, while the data processingsystem in FIG. 10 shows one processor, more processors may be presentdepending upon the implementation where each processor has a separatewake-and-go array or one wake-and-go array stores target addresses forthreads for multiple processors. In one illustrative embodiment, onewake-and-go engine stores entries in a central repository wake-and-goarray for all threads and multiple processors.

FIG. 23 is a block diagram illustrating a wake-and-go engine with acentral repository wake-and-go array in a multiple processor system inaccordance with an illustrative embodiment. Processors 2302-2308 connectto bus 2320. A device (not shown) may modify a data value at a targetaddress through input/output channel controller (IIOC) 2312, whichtransmits the transaction on bus 2320 to memory controller 2314.Wake-and-go engine 2350 performs look-ahead to identify wake-and-goprogramming idioms in the instruction streams of threads running onprocessors 2302-2308. If wake-and-go engine 2350 recognizes awake-and-go programming idiom, wake-and-go engine 2350 records an entryin central repository wake-and-go array 2352.

Wake-and-go engine 2350 snoops bus 2320. If a transaction appears on bus2320 that modifies a value at an address where the value satisfies thecomparison type in a wake-and-go array, then the wake-and-go engine 2350may wake a thread. Wake-and-go engine 2350 may have associated logicthat recognizes the target address on bus 2320 and performs thecomparison based on the value being written, the expected value storedin the wake-and-go array, and the comparison type stored in centralrepository wake-and-go array 2352. Thus, wake-and-go engine 2350 mayonly wake a thread if there is a transaction on bus 2320 that modifiesthe target address with a value that satisfies the comparison type andexpected value.

FIG. 24 illustrates a central repository wake-and-go-array in accordancewith an illustrative embodiment. Each entry in central repositorywake-and-go array 2400 may include thread identification (ID) 2402,central processing unit (CPU) ID 2404, the target address 2406, theexpected data 2408, a comparison type 2410, a lock bit 2412, a priority2414, and a thread state pointer 2416, which is the address at which thethread state information is stored.

The wake-and-go engine 2350 may use the thread ID 2402 to identify thethread and the CPU ID 2404 to identify the processor. Wake-and-go engine2350 may then place the thread in the run queue for the processoridentified by CPU ID 2404. Wake-and-go engine 2350 may also use threadstate pointer 2416 to load thread state information, which is used towake the thread to the proper state.

Programming Idiom Accelerator

In a sense, a wake-and-go mechanism, such as look-ahead wake-and-goengine 2350, is a programming idiom accelerator. A programming idiom isa sequence of programming instructions that occurs often and isrecognizable as a sequence of instructions. In the examples describedabove, an instruction sequence that includes load (LD), compare (CMP),and branch (BC) commands represents a programming idiom that indicatesthat the thread is waiting for data to be written to a particular targetaddress. Wake-and-go engine 2350 recognizes this idiom as a wake-and-goidiom and accelerates the wake-and-go process accordingly, as describedabove. Other examples of programming idioms may include spinning on alock or traversing a linked list.

FIG. 25 is a block diagram illustrating a programming idiom acceleratorin accordance with an illustrative embodiment. Processors 2502-2508connect to bus 2520. A processor, such as processor 2502 for example,may fetch instructions from memory via memory controller 2514. Asprocessor 2502 fetches instructions, programming idiom accelerator 2550may look ahead to determine whether a programming idiom is coming up inthe instruction stream. If programming idiom accelerator 2550 recognizesa programming idiom, programming idiom accelerator 2550 performs anaction to accelerate execution of the programming idiom. In the case ofa wake-and-go programming idiom, programming idiom accelerator 2550 mayrecord an entry in a wake-and-go array, for example.

As another example, if programming idiom accelerator 2550 accelerateslock spinning programming idioms, programming idiom accelerator 2550 mayobtain the lock for the processor, if the lock is available, thus makingthe lock spinning programming sequence of instructions unnecessary.Programming idiom accelerator 2550 may accelerate any known or commonsequence of instructions or future sequences of instructions. Althoughnot shown in FIG. 25, a data processing system may include multipleprogramming idiom accelerators that accelerate various programmingidioms. Alternatively, programming idiom accelerator 2550 may recognizeand accelerator multiple known programming idioms. In one exemplaryembodiment, each processor 2502-2508 may have programming idiomaccelerators within the processor itself.

As stated above with respect to the wake-and-go engine, programmingidiom accelerator 2550 may be a hardware device within the dataprocessing system. In an alternative embodiment, programming idiomaccelerator 2550 may be a hardware component within each processor2502-2508. In another embodiment, programming idiom accelerator 2550 maybe software within an operating system running on one or more ofprocessors 2502-2508. Thus, in various implementations or embodiments,programming idiom accelerator 2550 may be software, such as a backgroundsleeper thread or part of an operating system, hardware, or acombination of hardware and software.

In one embodiment, the programming language may include hintinstructions that may notify programming accelerator 2550 that aprogramming idiom is coming. FIG. 26 is a series of instructions thatare a programming idiom with programming language exposure in accordancewith an illustrative embodiment. In the example depicted in FIG. 26, theinstruction stream includes programming idiom 2602, which in this caseis an instruction sequence that includes load (LD), compare (CMP), andbranch (BC) commands that indicate that the thread is waiting for datato be written to a particular target address.

Idiom begin hint 2604 exposes the programming idiom to the programmingidiom accelerator. Thus, the programming idiom accelerator need notperform pattern matching or other forms of analysis to recognize asequence of instructions. Rather, the programmer may insert idiom hintinstructions, such as idiom begin hint 2604, to expose the idiom 2602 tothe programming idiom accelerator. Similarly, idiom end hint 2606 maymark the end of the programming idiom; however, idiom end hint 2606 maybe unnecessary if the programming idiom accelerator is capable ofidentifying the sequence of instructions as a recognized programmingidiom.

In an alternative embodiment, a compiler may recognize programmingidioms and expose the programming idioms to the programming idiomaccelerator. FIG. 27 is a block diagram illustrating a compiler thatexposes programming idioms in accordance with an illustrativeembodiment. Compiler 2710 receives high level program code 2702 andcompiles the high level instructions into machine instructions to beexecuted by a processor. Compiler 2710 may be software running on a dataprocessing system, such as data processing system 100 in FIG. 1, forexample.

Compiler 2710 includes programming idiom exposing module 2712, whichparses high level program code 2702 and identifies sequences ofinstructions that are recognized programming idioms. Compiler 2710 thencompiles the high level program code 2702 into machine instructions andinserts hint instructions to expose the programming idioms. Theresulting compiled code is machine code with programming idioms exposed2714. As machine code 2714 is fetched for execution by a processor, oneor more programming idiom accelerators may see a programming idiomcoming up and perform an action to accelerate execution.

FIG. 28 is a flowchart illustrating operation of a compiler exposingprogramming idioms in accordance with an illustrative embodiment.Operation begins and the compiler receives high level program code tocompile into machine code (block 2802). The compiler considers asequence of code (block 2804) and determines whether the sequence ofcode includes a recognized programming idiom (block 2806).

If the sequence of code includes a recognized programming idiom, thecompiler inserts one or more instructions to expose the programmingidiom to the programming idiom accelerator (block 2808). The compilercompiles the sequence of code (block 2810). If the sequence of code doesnot include a recognized programming idiom in block 2806, the compilerproceeds to block 2810 to compile the sequence of code.

After compiling the sequence of code in block 2810, the compilerdetermines if the end of the high level program code is reached (block2812). If the end of the program code is not reached, operation returnsto block 2804 to consider the next sequence of high level programinstructions. If the end of the program code is reached in block 2812,then operation ends.

The compiler may recognize one or more programming idioms from a set ofpredetermined programming idioms. The set of predetermined programmingidioms may correspond to a set of programming idiom accelerators thatare known to be supported in the target machine. For example, if thetarget data processing system has a wake-and-go engine and a linked listacceleration engine, then the compiler may provide hints for these twoprogramming idioms. The hint instructions may be such that they areignored by a processor or data processing system that does not supportprogramming idiom accelerators.

System Bus Response

Returning to FIG. 19, processors 1902-1908 connect to bus 1920. Each oneof processors 1902-1908 may have a wake-and-go mechanism, such aswake-and-go mechanism 1008 in FIG. 10, and a wake-and-go array, such aswake-and-go array 1022 in FIG. 10. A device (not shown) may modify adata value at a target address through input/output channel controller(HOC) 1912, which transmits the transaction on bus 1920 to memorycontroller 1914. That device may be a direct memory access (DMA) enginethat performs a data move operation.

The wake-and-go array of each processor 1902-1908 snoops bus 1920. If atransaction appears on bus 1920 that modifies a value at an addresswhere the value satisfies the comparison type in a wake-and-go array,then the wake-and-go mechanism may wake a thread. Each wake-and-go arraymay have associated logic that recognizes the target address on bus 1920and performs the comparison based on the value being written, theexpected value stored in the wake-and-go array, and the comparison typestored in the wake-and-go array. Thus, the wake-and-go mechanism mayonly wake a thread if there is a transaction on bus 1920 that modifiesthe target address with a value that satisfies the comparison type andexpected value.

A common sequence of instructions involves a direct memory access (DMA)operation. A processor may issue a command to a DMA engine to move ablock of data from one location, a source address, to another location,a target address. The DMA engine performs the DMA operation and issues acommand on the bus that notifies any waiting processes that the data isavailable at the target address. A process may wait for the command sothat the process may perform some work on the data at the targetaddress. Thus, the process may perform a load and compare withoutreservation to determine whether the data is ready and then perform aload and compare with reservation to obtain a lock on the data toperform work.

The process obtains a lock because two or more of processors 1902-1908may be attempting to perform work on the same block of data. One processmay obtain the lock while the others spin on the lock; then a nextprocess may obtain the lock, and so forth. All of this information iscommunicated over the bus using system bus response, also referred to assnoop response. That is, processors 1902-1908 communicate over bus 1920using system bus commands and/or system bus responses when they arereading data, writing data, waiting for data, have exclusive access todata, etc.

As described above, a look-ahead wake-and-go engine associated with agiven processor may perform a look-ahead polling operation. Thewake-and-go engine may perform a first polling operation to determine ifthe DMA engine has already moved the data and then perform a secondpolling operation to determine if the wake-and-go engine can obtain alock on the data. This would accelerate execution of the sequence ofinstructions, because when the sequence of instructions is executed, thewake-and-go engine may already have determined that the data is readyand obtained the lock. Other wake-and-go engines may already have puttheir threads to sleep and may have entries in the wake-and-go array to“spin” on the lock while the processor performs other work.

Because the wake-and-go engine performs reads and obtains locks, thewake-and-go engine must be capable of acting as a transaction master onthe bus and to generate a system bus response. FIG. 29 is a flowchartillustrating operation of a wake-and-go engine performing look-aheadpolling with system bus response in accordance with an illustrativeembodiment. Operation begins and the wake-and-go engine performs alook-ahead load without reservation (block 2902). This read determineswhether the DMA engine has completed a data move. The wake-and-go engineacts as a transaction master when reading the data, and other devices,including other wake-and-go engines, may generate system bus responsesas a result of the load operation.

The look-ahead load without reservation informs other wake-and-goengines that the wake-and-go engine wants to see the data, but does notwant exclusivity and will not cache the data. This may be an existingload without reservation, because wake-and-go engines may assume that aload without reservation is being performed by another wake-and-goengine. However, in an alternative embodiment, the bus architecture maysupport a specialized look-ahead load command to communicate that awake-and-go engine, or other device, is performing a look-ahead readwithout caching or exclusivity.

The wake-and-go engine receives system bus response from otherwake-and-go engines (block 2904). Then, the wake-and-go enginedetermines whether another wake-and-go engine has data exclusivity, alock on the data (block 2906). If another wake-and-go engine does nothave data exclusivity, then the wake-and-go engine performs thelook-ahead load without reservation to see the data for the pollingoperation and to perform a comparison. For the purpose of the pollingoperation, the wake-and-go engine only requires a lock on the data ifthe data value is the value for which the thread is waiting.Subsequently, the wake-and-go engine performs a compare operation todetermine whether the data value is the value for which the thread iswaiting (block 2908). Then, the wake-and-go engine determines whetherthe comparison results in a match (block 2910). Operation of awake-and-go mechanism with data monitoring is described above withreference to FIGS. 19 and 20.

If the comparison operation does not result in a match, then in thiscase the DMA engine has not yet completed the data move operation. Inthis case, or if another wake-and-go engine has data exclusivity inblock 2906, then the wake-and-go engine places an entry in thewake-and-go array (block 2912). Then, the wake-and-go engine snoops thetarget address without exclusivity (block 2914), and operation ends.When the DMA engine completes the data move, the wake-and-go engine willdetect the target address appearing on the address bus and wake thethread to continue execution and attempt to obtain a lock on the data.

If the comparison operation results in a match in block 2910, then theDMA engine has completed the data move operation, and the wake-and-goengine performs a load with reservation to obtain a lock on the data sothat the thread may perform work on the data (block 2916). Thewake-and-go engine determines whether it has obtained a lock on the data(block 2918). As discussed above, other devices, likely otherwake-and-go engines, may also be attempting to obtain a lock on thedata. If the wake-and-go engine has obtained the lock, operation simplyends and at the time the instruction sequence is executed, thewake-and-go engine will already have obtained the lock for theprocessor.

If the wake-and-go engine does not obtain the lock in block 2918, thewake-and-go engine places an entry in the wake-and-go array and sets thelock bit to spin on the lock (block 2920). Operation of a wake-and-gomechanism performing parallel lock spinning is described above withreference to FIGS. 21A, 21B, 22A, and 22B. Thereafter, the wake-and-goengine snoops the lock address and requests data exclusivity (block2922), and operation ends.

Thus, the wake-and-go engine provides one of two different system busresponses based on the content of the data. The wake-and-go enginerequests data exclusivity if the data value is the value for which thethread is waiting, and does not request exclusivity if the data value isnot the expected value.

FIG. 30 is a flowchart illustrating operation of a wake-and-go engineperforming a snoop operation without data exclusivity in accordance withan illustrative embodiment. Operation begins, and the wake-and-go enginedetermines if a target address appears on the address bus (block 3002).If the target address does not appear on the address bus, then operationreturns to block 3002 to wait for the target address to appear on theaddress bus.

If the target address does appear on the address bus in block 3002, thewake-and-go engine performs a load without reservation (block 3004). Thewake-and-go engine performs the look-ahead load without reservation tosee the data for the polling operation and to perform a comparison.Subsequently, the wake-and-go engine performs a compare operation todetermine whether the data value is the value for which the thread iswaiting (block 3006). Then, the wake-and-go engine determines whetherthe comparison results in a match (block 3008).

If the result of the comparison is not a match, operation returns toblock 3002 to wait for the target address to appear on the address bus.If the result of the comparison is a match in block 3008, operationproceeds to block 2912 in FIG. 29 to perform a load with reservation sothat the processor may obtain a lock on the data and perform work.

FIG. 31 is a flowchart illustrating operation of snoop response logic onthe system bus in accordance with an illustrative embodiment. Operationbegins, and the snoop response logic collects system bus responses fromwake-and-go engines for a given target address, where each of thewake-and-go engines request data exclusivity (block 3102). The snoopresponse logic determines which wake-and-go engine obtains the lock(block 3104). Then, the snoop response logic generates a combined snoopresponse (block 3106) and sends the combined snoop response to thewake-and-go engines (block 3108).

The winning wake-and-go engine may then hold the lock until the sequenceof instructions in the thread running on the associated processorreaches execution and works on the data. When the winning wake-and-goengine finishes working on the data, the winning wake-and-go engine mayrelease the lock. The remaining wake-and-go engines contending for thelock may place an entry in their respective wake-and-go arrays to spinon the lock.

The snoop response logic then determines whether the winning wake-and-goreleases the lock on the data (block 3110). If the winning wake-and-goengine does not release the lock on the data, then operation returns toblock 3110 until the winning wake-and-go engine releases the lock. Ifthe winning wake-and-go engine releases the lock in block 3110, thesnoop response logic generates a combines snoop response indicating thatthe winning wake-and-go engine released the lock (block 3112), and sendsthe combined snoop response on the bus (block 3114). Thereafter,operation ends.

Thus, the illustrative embodiments solve the disadvantages of the priorart by providing a wake-and-go mechanism for a microprocessor. When athread is waiting for an event, rather than performing a series ofget-and-compare sequences, the thread updates a wake-and-go array with atarget address associated with the event. The target address may pointto a memory location at which the thread is waiting for a value to bewritten. The thread may update the wake-and-go array using a processorinstruction within the program, a call to the operating system, or acall to a background sleeper thread, for example. The thread then goesto sleep until the event occurs.

The wake-and-go array may be a content addressable memory (CAM). When atransaction appears on the symmetric multiprocessing (SMP) fabric thatmodifies the value at a target address in the CAM, which is referred toas a “kill,” the CAM returns a list of storage addresses at which thetarget address is stored. The operating system or a background sleeperthread associates these storage addresses with the threads waiting foran even at the target addresses, and may wake the one or more threadswaiting for the event.

It should be appreciated that the illustrative embodiments may take theform of a specialized hardware embodiment, a software embodiment that isexecuted on a computer system having general processing hardware, or anembodiment containing both specialized hardware and software elementsthat are executed on a computer system having general processinghardware. In one exemplary embodiment, the mechanisms of theillustrative embodiments are implemented in a software product, whichmay include but is not limited to firmware, resident software,microcode, etc.

Furthermore, the illustrative embodiments may take the form of acomputer program product accessible from a computer-usable orcomputer-readable medium providing program code for use by or inconnection with a computer or any instruction execution system. For thepurposes of this description, a computer-usable or computer-readablemedium can be any apparatus that can contain, store, communicate,propagate, or transport the program for use by or in connection with theinstruction execution system, apparatus, or device.

The medium may be an electronic, magnetic, optical, electromagnetic, orsemiconductor system, apparatus, or device. Examples of acomputer-readable medium include a semiconductor or solid state memory,magnetic tape, a removable computer diskette, a random access memory(RAM), a read-only memory (ROM), a rigid magnetic disk, and an opticaldisk. Current examples of optical disks include compact disk-read-onlymemory (CD-ROM), compact disk-read/write (CD-R/W) and DVD.

The program code of the computer program product may compriseinstructions that are stored in a computer readable storage medium in aclient or server data processing system. In a client data processingsystem embodiment, the instructions may have been downloaded over anetwork from one or more remote data processing systems, such as aserver data processing system, a client data processing system, or aplurality of client data processing systems using a peer-to-peercommunication methodology. In a server data processing systemembodiment, the instructions may be configured for download, or actuallydownloaded, over a network to a remote data processing system, e.g., aclient data processing system, for use in a computer readable storagemedium with the remote data processing system.

A data processing system suitable for storing and/or executing programcode will include at least one processor coupled directly or indirectlyto memory elements through a system bus. The memory elements can includelocal memory employed during actual execution of the program code, bulkstorage, and cache memories which provide temporary storage of at leastsome program code in order to reduce the number of times code must beretrieved from bulk storage during execution.

Input/output or I/O devices (including but not limited to keyboards,displays, pointing devices, etc.) can be coupled to the system eitherdirectly or through intervening I/O controllers. Network adapters mayalso be coupled to the system to enable the data processing system tobecome coupled to other data processing systems or remote printers orstorage devices through intervening private or public networks. Modems,cable modems and Ethernet cards are just a few of the currentlyavailable types of network adapters.

The description of the present invention has been presented for purposesof illustration and description, and is not intended to be exhaustive orlimited to the invention in the form disclosed. Many modifications andvariations will be apparent to those of ordinary skill in the art. Theembodiment was chosen and described in order to best explain theprinciples of the invention, the practical application, and to enableothers of ordinary skill in the art to understand the invention forvarious embodiments with various modifications as are suited to theparticular use contemplated.

What is claimed is:
 1. A method, in a data processing system, forperforming a look-ahead operation in a wake-and-go mechanism, the methodcomprising: responsive to the wake-and-go mechanism detecting a sequenceof instructions in a thread that indicates the thread is waiting for anevent associated with a target address, issuing a look-ahead loadcommand on a system bus to read a data value from the target address;performing a comparison operation to determine whether the data value atthe target address indicates that the event, for which the thread iswaiting has occurred; in response to the comparison resulting in adetermination that the event has not occurred, populating a wake-and-gostorage array with the target address; and snooping the target addresson the system bus.
 2. The method of claim 1, wherein the look-ahead loadcommand is a load without reservation command.
 3. The method of claim 1,wherein the look-ahead load command is a specialized bus commandindicating that the wake-and-go mechanism is performing a look-aheadpolling operation without caching or data exclusivity.
 4. The method ofclaim 1, further comprising: in response to the comparison resulting ina determination that the event has occurred, issuing a load withreservation command on the system bus.
 5. The method of claim 1, whereinthe wake-and-go storage array is a content addressable memory and anaddress appearing on an address bus is used to address the contentaddressable memory.
 6. The method of claim 5, wherein responsive todetecting the event, the content addressable memory returns a storageaddress at which the target address is stored in the content addressablememory.
 7. The method of claim 1, wherein the event is a direct memoryaccess operation that moves data from a source address to the targetaddress.
 8. A data processing system, comprising: a wake-and-gomechanism; and a wake-and-go storage array, wherein the wake-and-gomechanism is configured to: responsive to the wake-and-go mechanismdetecting a sequence of instructions in a thread that indicates thethread is waiting for an event associated with a target address, issue alook-ahead load command on a system bus to read a data value from thetarget address; perform a comparison operation to determine whether thedata value at the target address indicates that the event for which thethread is waiting has occurred; in response to the comparison resultingin a determination that the event has not occurred, populate thewake-and-go storage array with the target address; and snoop the targetaddress on the system bus.
 9. The data processing system of claim 8,wherein the look-ahead load command is a load without reservationcommand.
 10. The data processing system of claim 8, wherein thelook-ahead load command is a specialized bus command indicating that thewake-and-go mechanism is performing a look-ahead polling operationwithout caching or data exclusivity.
 11. The data processing system ofclaim 8, wherein the wake-and-go mechanism is further configured to: inresponse to the comparison resulting in a determination that the eventhas occurred, issue a load with reservation command on the system bus.12. The data processing system of claim 8, wherein the wake-and-gostorage array is a content addressable memory and an address appearingon an address bus is used to address the content addressable memory. 13.The data processing system of claim 12, wherein responsive to detectingthe event, the content addressable memory returns a storage address atwhich the target address is stored in the content addressable memory.14. The data processing system of claim 8, wherein the event is a directmemory access operation that moves data from a source address to thetarget address.
 15. A computer program product comprising a computeruseable storage medium having a computer readable program storedthereon, wherein the computer readable program, when executed on acomputing device, causes the computing device to: responsive to thewake-and-go mechanism detecting a sequence of instructions in a threadthat indicates the thread is waiting for an event associated with atarget address, issue a look-ahead load command on a system bus to reada data value from the target address; perform a comparison operation todetermine whether the data value at the target address indicates thatthe event for which the thread is waiting has occurred; in response tothe comparison resulting in a determination that the event has notoccurred, populate a wake-and-go storage array with the target address;and snoop the target address on the system bus.
 16. The computer programproduct of claim 15, wherein the look-ahead load command is a loadwithout reservation command.
 17. The computer program product of claim15, wherein the look-ahead load command is a specialized bus commandindicating that the wake-and-go mechanism is performing a look-aheadpolling operation without caching or data exclusivity.
 18. The computerprogram product of claim 15, wherein the computer readable programfurther causes the computing device to: in response to the comparisonresulting in a determination that the event has occurred, issue a loadwith reservation command on the system bus.
 19. The computer programproduct of claim 15, wherein the wake-and-go storage array is a contentaddressable memory and an address appearing on an address bus is used toaddress the content addressable memory.
 20. The computer program productof claim 15, wherein the event is a direct memory access operation thatmoves data from a source address to the target address.
 21. A dataprocessing system, comprising: a wake-and-go mechanism; and a contentaddressable memory, wherein the wake-and-go mechanism is configured to:responsive to the wake-and-go mechanism detecting a sequence ofinstructions in a thread that indicates the thread is waiting for anevent associated with a target address, issue a look-ahead load commandon a system bus to read a data value from the target address todetermine whether the data value at the target address indicates thatthe event for which the thread is waiting has occurred: in response to adetermination that the event has not occurred, populate the contentaddressable memory with the target address; and snoop the target addresson the system bus, wherein an address appearing on the system bus isused to address the content addressable memory.